Andrei M Sanov

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• Dauletyarov, Y., Dixon, A. R., Wallace, A. A., & Sanov, A. (2017). Electron affinity and excited states of methylglyoxal. JOURNAL OF CHEMICAL PHYSICS, 147(1).
• Dixon, A. R., Xue, T., & Sanov, A. (2016). HOCCO versus OCCO: Comparative spectroscopy of the radical and diradical reactive intermediates. JOURNAL OF CHEMICAL PHYSICS, 144(23).
• Xue, T., Dixon, A. R., & Sanov, A. (2016). Anion photoelectron imaging spectroscopy of glyoxal. CHEMICAL PHYSICS LETTERS, 660, 205-208.
• Culberson, L. M., Blackstone, C. C., Wallace, A. A., & Sanov, A. (2015). Aromatic Stabilization and Hybridization Trends in Photoelectron Imaging of Heterocyclic Radicals and Anions. JOURNAL OF PHYSICAL CHEMISTRY A, 119(38), 9770-9777.
• Dixon, A. R., Khuseynov, D., & Sanov, A. (2015). Benzonitrile: Electron affinity, excited states, and anion solvation. JOURNAL OF CHEMICAL PHYSICS, 143(13).
• Dixon, A. R., Xue, T., & Sanov, A. (2015). Spectroscopy of Ethylenedione. ANGEWANDTE CHEMIE-INTERNATIONAL EDITION, 54(30), 8764-8767.
• Culberson, L. M., Blackstone, C. C., Wysocki, R., & Sanov, A. (2014). Selective deprotonation of oxazole and photoelectron imaging of the oxazolide anion. PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 16(2), 527-532.
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  A photoelectron imaging study of the oxazolide anion obtained by selective deprotonation of oxazole at the C2 position is reported. The photodetachment transitions observed at 355, 392, and 532 nm are assigned to the ground state of the neutral oxazolyl sigma radical. A Franck-Condon analysis of this transition aids in the first determination of the adiabatic electron affinity of oxazolyl, EA = 2.21 +/- 0.02 eV. A vibrational progression with a frequency of 890 +/- 80 cm(-1) is observed, corresponding to an in-plane ring distortion mode. The photoelectron angular distributions are analyzed using the mixed s-p model, shedding light on the hybrid character of the anion HOMO.
• Culberson, L. M., Wallace, A. A., Blackstone, C. C., Khuseynov, D., & Sanov, A. (2014). Spectroscopy of the breaking bond: the diradical intermediate of the ring opening in oxazole. PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 16(9), 3964-3972.
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  Bond breaking is a challenging problem in both experimental and theoretical chemistry, due to the transient nature and multi-configurational electronic structure of dissociating molecules. We use anion photodetachment to probe the diradical interactions in the ring-opening reaction of oxazole and obtain a self-consistent picture of the breaking bond. Starting from the closed-shell cyclic molecule, the reaction is launched on the anion potential, as an attached electron cleaves a carbon-oxygen bond. In the photodetachment, two neutral potential regions are sampled. One corresponds to a completely dissociated bond, while the other - to the bond fragments separated by approximately 3 angstrom. At this chemically relevant distance, signatures of lingering through-space interactions between the radical centers are observed.
• Dixon, A. R., Khuseynov, D., & Sanov, A. (2014). Heterogeneously Substituted Radicals and Carbenes: Photoelectron Imaging of the FC(H)CN- and FCCN- Anions. JOURNAL OF PHYSICAL CHEMISTRY A, 118(37), 8533-8541.
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  This work represents the next step in the studies of heterogeneous substitution effects in cyanohalo radicals and carbenes. Negative-ion photoelectron imaging was used to investigate the substituted radical and carbene derivatives of fluoroacetonitrile. We report a closed-shell singlet ground state for cyanofluorocarbene, FCCN, with a directly measured adiabatic electron affinity EA = 2.081 +/- 0.002 eV and a singlet triplet gap of Delta ES-T = 0.42 +/- 0.04 eV, estimated through a combination of experimental and theoretical results. The open-shell singlet (1)A '' state was also observed experimentally. The cyanofluoromethyl radical, FC(H)CN, was similarly estimated to have an EA of 1.53 +/- 0.08 eV. This value was used to estimate the C-H bond dissociation energy (BDE) of fluoroacetonitrile, DH298 = 90.7 +/- 2.8 kcal mol(-1). The results are discussed in comparison with results for other fluoro- and cyano-substituted radicals and carbenes, and in light of our recent work on the radical and carbene derivatives of chloroacetonitrile. The estimated Delta ES-T of FCCN agrees well with the general trend of similar carbenes. We also find that, similar to chloroacetonitrile, the low C-H BDE of fluoroaceotnitrile indicates a synergistic stabilization of the corresponding radical by a pi donor (halogen) and pi acceptor (CN).
• Dixon, A. R., Khuseynoy, D., & Sanov, A. (2014). Cyanobenzyl and chlorobenzyl radicals via anion photoelectron imaging spectroscopy. CHEMICAL PHYSICS LETTERS, 614, 72-77.
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  We report the adiabatic electron affinity of the cyanobenzyl radical, EA(PhCHCN) = 1.90 +/- 0.01 eV, determined from a combination of the experiment and theory, and assign an upper limit of the EA for the chlorobenzyl radical, EA(PhCHC1)
• Khuseynov, D., Blackstone, C. C., Culberson, L. M., & Sanov, A. (2014). Photoelectron angular distributions for states of any mixed character: An experiment-friendly model for atomic, molecular, and cluster anions. JOURNAL OF CHEMICAL PHYSICS, 141(12).
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  We present a model for laboratory-frame photoelectron angular distributions in direct photodetachment from (in principle) any molecular orbital using linearly polarized light. A transparent mathematical approach is used to generalize the Cooper-Zare central-potential model to anionic states of any mixed character. In the limit of atomic-anion photodetachment, the model reproduces the Cooper-Zare formula. In the case of an initial orbital described as a superposition of s and p-type functions, the model yields the previously obtained s-p mixing formula. The formalism is further advanced using the Hanstorp approximation, whereas the relative scaling of the partial-wave cross-sections is assumed to follow the Wigner threshold law. The resulting model describes the energy dependence of photoelectron anisotropy for any atomic, molecular, or cluster anions, usually without requiring a direct calculation of the transition dipole matrix elements. As a benchmark case, we apply the p-d variant of the model to the experimental results for NO- photodetachment and show that the observed anisotropy trend is described well using physically meaningful values of the model parameters. Overall, the presented formalism delivers insight into the photodetachment process and affords a new quantitative strategy for analyzing the photoelectron angular distributions and characterizing mixed-character molecular orbitals using photoelectron imaging spectroscopy of negative ions. (C) 2014 AIP Publishing LLC.
• Khuseynov, D., Dixon, A. R., Dokuchitz, D. J., & Sanov, A. (2014). Photochemistry of Fumaronitrile Radical Anion and Its Clusters. JOURNAL OF PHYSICAL CHEMISTRY A, 118(25), 4510-4518.
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  The photodetachment and photochemistry of the radical anion of fumaronitrile (trans-1,2-dicyanoethylene) and its clusters are investigated using photoelectron imaging and photofragment spectroscopy. We report the first direct spectroscopic determination of the adiabatic electron affinity (EA) of fumaronitrile (fn) in the gas phase, EA = 1.21 +/- 0.02 eV. This is significantly smaller than one-half the EA of tetracyanoethylene (TCNE). The singlet-triplet splitting in fumaronitrile is determined to be Delta ES-T
• Khuseynov, D., Stanton, J. F., & Sanov, A. (2014). Low-Lying Electronic States of Cyclopentadienone. JOURNAL OF PHYSICAL CHEMISTRY A, 118(34), 6965-6970.
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  We report a combined experimental and theoretical study of the low-lying electronic states of cyclopentadienone (C5H4O). The cyclopentadienone anion (C5H4O-) was generated in the gas phase via reaction of atomic oxygen radical anions (O-) with cyclopentanone (C5H8O). Photoelectron imaging was used to gain access to the first three electronic states of C5H4O, including the X(1)A(1) ground state and the B-3(2) and (3)A(2) excited states. The first two state assignments are supported by the Franck-Condon simulations of the vibrational progressions observed in the X (1)A(1), and B-3(2) bands in the photoelectron spectra. The adiabatic electron affinity of cyclopentadienone in the ground state is determined to be EA(X (1)A(1)) = 1.06 +/- 0.01 eV, and the corresponding values for the first two excited states are EA(B-3(2)) = 2.56 +/- 0.02 eV and EA((3)A(2)) = 3.45 +/- 0.01 eV. These experimental determinations are in excellent agreement with the CCSD(T) theory predictions, lending further confidence to the above state assignments. On the basis of these results, the lowest singlet-triplet splitting (between the X (1)A(1) and B-3(2) states) in cydopentadienone is Delta ES-T = 1.50 +/- 0.02 eV.
• Sanov, A. (2014). Laboratory-Frame Photoelectron Angular Distributions in Anion Photodetachment: Insight into Electronic Structure and Intermolecular Interactions. ANNUAL REVIEWS OF PHYSICAL CHEMISTRY, 65, 341-363.
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  This article provides an overview of some recent advances in the modeling of photoelectron angular distributions in negative-ion photodetachment. Building on the past developments in threshold photodetachment spectroscopy that first tackled the scaling of the partial cross sections with energy, depending on the angular momentum quantum number l, it examines the corresponding formulation of the central potential model and extends it to the more general case of hybrid molecular orbitals. Several conceptual approaches to understanding photoelectron angular distributions are discussed. In one approach, the angular distributions are examined based on the contributions of the symmetry-allowed s and p partial waves of the photodetached electron. In another related approach, the parent molecular orbitals are described based on their dominant s and p characters, whereas the continuum electron is described in terms of interference of the corresponding Delta l = +/- 1 photodetachment channels.
• Culberson, L. M., Blackstone, C. C., & Sanov, A. (2013). Photoelectron angular distributions of pyridinide: A benchmark application of the mixed s-p model to a truly polyatomic anion. Journal of Physical Chemistry A, 117(46), 11760-11765.
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  PMID: 24256190;Abstract: We report a photoelectron imaging study of the pyridinide anion, C 5H4N-, obtained by deprotonation of pyridine at the C4 position. The photoelectron angular distributions are used to test the theoretical formalism for photodetachment from mixed-character s-p states, demonstrating its first application to a truly polyatomic system. The mixed s-p model describes the initial state of the anion in terms of a superposition of one s orbital and one p orbital centered on the deprotonated carbon. Using the model parameter values determined from ab initio calculations, without any fits to the experimental data, the theory yields good quantitative agreement to the experiment. The agreement is demonstrated using either the canonical Hartree-Fock highest-occupied molecular orbital of the anion or the corresponding Dyson orbital. The results confirm the predictive power of the mixed s-p model and suggest that despite its approximate nature it captures the essential physics of the photoemission process. © 2013 American Chemical Society.
• Khuseynov, D., Dixon, A. R., Goebbert, D. J., & Sanov, A. (2013). Heterogeneous substitution effects in chlorocyanomethyl radical and chlorocyanocarbene. Journal of Physical Chemistry A, 117(41), 10681-10691.
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  PMID: 24050499;Abstract: We report a photoelectron-imaging investigation of the chlorocyanomethyl radical (CHClCN) and the corresponding carbene (CClCN). The results are discussed in comparison with the corresponding dichloro- and dicyano-substituted species, focusing on the divergent effects of the halogen and pseudohalogen (CN) substitutions. A cooperative (captodative) interaction of the π-donor Cl and π-acceptor cyano groups favors the increased stability of the CHClCN radical, but a competition of the two substituents is observed in the singlet-triplet splitting of the carbene. The vertical detachment energy (VDE) of CHClCN- is determined to be 2.39 ± 0.04 eV, with the broad photoelectron band consistent with the significant geometry change predicted by theory for the detachment transition. The adiabatic electron affinity of CHClCN, EA = 1.86 ± 0.08 eV, is estimated on the basis of the experimental VDE and the computed difference between the VDE and EA values. This result allows the calculation of the bond dissociation energy of chloroacetonitrile, DH 298(H-CHClCN) = 87.0 ± 2.7 kcal/mol. Photoelectron imaging of CClCN- reveals two main transitions, assigned to the singlet ( 1A′) and triplet (3A″) states of the CClCN carbene. The respective VDEs are 2.76 ± 0.05 and 3.25 ± 0.05 eV. The experimental results are in good agreement with the theoretically predicted singlet-triplet vertical energy gap at the anion geometry, but inconclusive with regard to the adiabatic singlet-triplet splitting in CClCN. Consistent with the experimental findings, ab initio calculations using the spin-flip approach in combination with the coupled-cluster theory, indicate that the 1A′ and 3A″ states are nearly degenerate, with the singlet state lying adiabatically only ∼0.01 eV below the triplet. © 2013 American Chemical Society.
• Sanov, A., Culberson, L. M., Blackstone, C. C., Wysocki, R., & Sanov, A. M. (2013). Selective deprotonation of oxazole and photoelectron imaging of the oxazolide anion. Physical chemistry chemical physics : PCCP.
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  A photoelectron imaging study of the oxazolide anion obtained by selective deprotonation of oxazole at the C2 position is reported. The photodetachment transitions observed at 355, 392, and 532 nm are assigned to the ground state of the neutral oxazolyl σ radical. A Franck-Condon analysis of this transition aids in the first determination of the adiabatic electron affinity of oxazolyl, EA = 2.21 ± 0.02 eV. A vibrational progression with a frequency of 890 ± 80 cm(-1) is observed, corresponding to an in-plane ring distortion mode. The photoelectron angular distributions are analyzed using the mixed s-p model, shedding light on the hybrid character of the anion HOMO.
• Sanov, A., Grumbling, E. R., Goebbert, D. J., & Culberson, L. M. (2013). Photodetachment anisotropy for mixed s-p states: 8/3 and other fractions. Journal of Chemical Physics, 138(5).
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  PMID: 23406123;Abstract: An approximate model for analytical prediction of photoelectron angular distributions in anion photodetachment from mixed s-p states is presented. Considering the dipole-allowed s, p, and d free-electron partial waves, the model describes photodetachment anisotropy in terms of the fractional p character of the initial orbital and the A and B coefficients describing the relative intensities of the p → d to p → s and s → p to p → s channels, respectively. The model represents an extension of the central-potential model to an intermediate regime encompassing varying degrees of s and p contributions to the initial bound orbital. This description is applicable to a broad class of hybrid molecular orbitals, particularly those localized predominantly on a single atom. Under the additional assumption of hydrogenic or Slater-type orbitals, the B/A ratio in photodetachment from a mixed 2s-2p state is shown to equal 8/3. Corresponding fractions are derived for other ns-np mixing cases. The predictions of the model are tested on several anion systems, including NH2- and CCl2-. The quantitative discrepancies in the latter case are attributed to the breakdown of the central-atom approximation and a mechanism for corresponding corrections is indicated. © 2013 American Institute of Physics.
• Sanov, A., Khuseynov, D., Dixon, A. R., Goebbert, D. J., & Sanov, A. M. (2013). Heterogeneous substitution effects in chlorocyanomethyl radical and chlorocyanocarbene. The journal of physical chemistry. A, 117(41).
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  We report a photoelectron-imaging investigation of the chlorocyanomethyl radical (CHClCN) and the corresponding carbene (CClCN). The results are discussed in comparison with the corresponding dichloro- and dicyano-substituted species, focusing on the divergent effects of the halogen and pseudohalogen (CN) substitutions. A cooperative (captodative) interaction of the π-donor Cl and π-acceptor cyano groups favors the increased stability of the CHClCN radical, but a competition of the two substituents is observed in the singlet-triplet splitting of the carbene. The vertical detachment energy (VDE) of CHClCN(-) is determined to be 2.39 ± 0.04 eV, with the broad photoelectron band consistent with the significant geometry change predicted by theory for the detachment transition. The adiabatic electron affinity of CHClCN, EA = 1.86 ± 0.08 eV, is estimated on the basis of the experimental VDE and the computed difference between the VDE and EA values. This result allows the calculation of the bond dissociation energy of chloroacetonitrile, DH298(H-CHClCN) = 87.0 ± 2.7 kcal/mol. Photoelectron imaging of CClCN(-) reveals two main transitions, assigned to the singlet ((1)A') and triplet ((3)A″) states of the CClCN carbene. The respective VDEs are 2.76 ± 0.05 and 3.25 ± 0.05 eV. The experimental results are in good agreement with the theoretically predicted singlet-triplet vertical energy gap at the anion geometry, but inconclusive with regard to the adiabatic singlet-triplet splitting in CClCN. Consistent with the experimental findings, ab initio calculations using the spin-flip approach in combination with the coupled-cluster theory, indicate that the (1)A' and (3)A″ states are nearly degenerate, with the singlet state lying adiabatically only ∼0.01 eV below the triplet.
• Khuseynov, D., Fontana, M. T., & Sanov, A. (2012). Photoelectron spectroscopy and photochemistry of tetracyanoethylene radical anion in the gas phase. Chemical Physics Letters, 550, 15-18.
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  Abstract: A combined photoelectron spectroscopy, photofragmentation and clustering study of the tetracyanoethylene (TCNE) radical anion in the gas phase is reported. The adiabatic electron affinity of TCNE is 3.16(2) eV. This result represents an order-of-magnitude improvement in the uncertainty over the presently accepted, indirectly determined value. The photodissociation of the anion at 355 and 266 nm leads to three anionic channels: one yielding CN - and the other two corresponding to the loss of one or two CN groups from the parent anion. The CN - fragment channel is dominant at 355 nm, while the double-dissociation channel dominates 266 nm photochemistry. © 2012 Elsevier B.V. All rights reserved.
• Khuseynov, D., Goebbert, D. J., & Sanov, A. (2012). Oxygen cluster anions revisited: Solvent-mediated dissociation of the core O ₄^- anion. Journal of Chemical Physics, 136(9).
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  PMID: 22401445;Abstract: The electronic structure and photochemistry of the O 2n-(H 2O) m, n 1-6, m 0-1 cluster anions is investigated at 532 nm using photoelectron imaging and photofragment mass-spectroscopy. The results indicate that both pure oxygen clusters and their hydrated counterparts with n 2 form an O 4- core. Fragmentation of these clusters yields predominantly O 2- and O 2-H 2O anionic products, with the addition of O 4- fragments for larger parent clusters. The fragment autodetachment patterns observed for O 6- and larger O 2n- species, as well as some of their hydrated counterparts, indicate that the corresponding O 2- fragments are formed in excited vibrational states (v 4). Yet, surprisingly, the unsolvated O 4- anion itself does not show fragment autodetachment at 532 nm. It is hypothesized that the vibrationally excited O 2- is formed in the intra-cluster photodissociation of the O 4- core anion via a charge-hopping electronic relaxation mechanism mediated by asymmetric solvation of the nascent photofragments: O 4- → O 2-(X 2g) O 2(a 1g) → O 2(X 3g-) O 2-(X 2g). This process depends on the presence of solvent molecules and leads to vibrationally excited O 2-(X 2g) products. © 2012 American Institute of Physics.
• Sanov, A., Khuseynov, D., Goebbert, D. J., & Sanov, A. M. (2012). Oxygen cluster anions revisited: solvent-mediated dissociation of the core O4(-) anion. The Journal of chemical physics, 136(9).
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  The electronic structure and photochemistry of the O(2n)(-)(H(2)O)(m), n = 1-6, m = 0-1 cluster anions is investigated at 532 nm using photoelectron imaging and photofragment mass-spectroscopy. The results indicate that both pure oxygen clusters and their hydrated counterparts with n ≥ 2 form an O(4)(-) core. Fragmentation of these clusters yields predominantly O(2)(-) and O(2)(-)·H(2)O anionic products, with the addition of O(4)(-) fragments for larger parent clusters. The fragment autodetachment patterns observed for O(6)(-) and larger O(2n)(-) species, as well as some of their hydrated counterparts, indicate that the corresponding O(2)(-) fragments are formed in excited vibrational states (v ≥ 4). Yet, surprisingly, the unsolvated O(4)(-) anion itself does not show fragment autodetachment at 532 nm. It is hypothesized that the vibrationally excited O(2)(-) is formed in the intra-cluster photodissociation of the O(4)(-) core anion via a charge-hopping electronic relaxation mechanism mediated by asymmetric solvation of the nascent photofragments: O(4)(-) → O(2)(-)(X(2)Π(g)) + O(2)(a(1)Δ(g)) → O(2)(X(3)Σ(g)(-)) + O(2)(-)(X(2)Π(g)). This process depends on the presence of solvent molecules and leads to vibrationally excited O(2)(-)(X(2)Π(g)) products.
• Sanov, A., Smith, M. A., Yuan, B., & Sanov, A. M. (2012). Variable temperature rate studies for the reaction H3O(+) + (C2H2)2 measured with a coaxial molecular beam radio frequency ring electrode ion trap. The journal of physical chemistry. A, 116(38).
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  The independent molecule and ion temperature dependence of the rate coefficient for the H(3)O(+) and (C(2)H(2))(2) reaction producing C(2)H(5)O(+) are determined using a coaxial molecular beam radio frequency ring electrode ion trap (CoMB-RET). The H(3)O(+) temperature is varied from 25 to 170 K, while the equilibrated C(2)H(2)/(C(2)H(2))(2) beam temperatures sampled are 160, 180, 200, and 220 K. The rate coefficient of the H(3)O(+) + (C(2)H(2))(2) reaction is determined to be 4.0 × 10(-10) × (T(react)/300)(-2.5) in the reaction temperature range of T(react) = 114-187 K. The H(3)O(+) and C(2)H(2) radiative association reaction is found to have a rate coefficient below 1 × 10(-13) cm(3)·s(-1) at 187 K. This result is consistent with Herbst's experimental determination.
• Sanov, A., Smith, M. A., Yuan, B., & Sanov, A. M. (2012). Variable-temperature rate coefficients of proton-transfer equilibrium reaction C2H4 + H3O+ ⇄ C2H5+ + H2O measured with a coaxial molecular beam radio frequency ring electrode ion trap. The journal of physical chemistry. A, 116(47).
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  The rate coefficients for the forward and reverse proton-transfer reactions C(2)H(4) + H(3)O(+) ⇄ C(2)H(5)(+) + H(2)O are studied with respect to independent varied neutral molecule and ion temperatures. The measurements are performed using a coaxial molecular beam radio frequency ring electrode ion trap at trap temperatures down to 23 K and beam temperatures up to 450 K. The temperature-dependent rate coefficients suggest that in this temperature window, the reaction proceeds through a statistically equilibrated complex. In order to explain the observed rate coefficients, a new type of reaction temperature was defined in these studies that considered collisional and internal (rotational and vibrational) degrees of freedom of both H(3)O(+) and C(2)H(4). The enthalpy and entropy of the equilibrium reaction deduced from a Van't Hoff plot are ΔH = (5.1 ± 0.5) kJ·mol(-1) and ΔS = (-15.0 ± 0.9) J·mol(-1)·K(-1), respectively.
• Smith, M. A., Yuan, B., & Sanov, A. (2012). Variable temperature rate studies for the reaction H₃O ⁺ + (C₂H₂)₂ measured with a coaxial molecular beam radio frequency ring electrode ion trap. Journal of Physical Chemistry A, 116(38), 9466-9472.
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  PMID: 22946680;Abstract: The independent molecule and ion temperature dependence of the rate coefficient for the H3O+ and (C2H 2)2 reaction producing C2H5O + are determined using a coaxial molecular beam radio frequency ring electrode ion trap (CoMB-RET). The H3O+ temperature is varied from 25 to 170 K, while the equilibrated C2H 2/(C2H2)2 beam temperatures sampled are 160, 180, 200, and 220 K. The rate coefficient of the H3O + + (C2H2)2 reaction is determined to be 4.0 × 10-10 × (Treact/300) -2.5 in the reaction temperature range of Treact = 114-187 K. The H3O+ and C2H2 radiative association reaction is found to have a rate coefficient below 1 × 10 -13 cm3·s-1 at 187 K. This result is consistent with Herbst's experimental determination. © 2012 American Chemical Society.
• Smith, M. A., Yuan, B., & Sanov, A. (2012). Variable-temperature rate coefficients of proton-transfer equilibrium reaction C₂H₄ + H₃O⁺ ↔ C ₂H₅⁺ + H₂O measured with a coaxial molecular beam radio frequency ring electrode ion trap. Journal of Physical Chemistry A, 116(47), 11596-11600.
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  PMID: 23145801;Abstract: The rate coefficients for the forward and reverse proton-transfer reactions C2H4 + H3O+ ↔ C 2H5+ + H2O are studied with respect to independent varied neutral molecule and ion temperatures. The measurements are performed using a coaxial molecular beam radio frequency ring electrode ion trap at trap temperatures down to 23 K and beam temperatures up to 450 K. The temperature-dependent rate coefficients suggest that in this temperature window, the reaction proceeds through a statistically equilibrated complex. In order to explain the observed rate coefficients, a new type of reaction temperature was defined in these studies that considered collisional and internal (rotational and vibrational) degrees of freedom of both H3O+ and C2H4. The enthalpy and entropy of the equilibrium reaction deduced from a Van't Hoff plot are δH = (5.1 ± 0.5) kJ̇mol-1 and δS = (-15.0 ± 0.9) J̇mol -1̇K-1, respectively. © 2012 American Chemical Society.
• Culberson, L. M., & Sanov, A. (2011). Electronic states of thiophenyl and furanyl radicals and dissociation energy of thiophene via photoelectron imaging of negative ions. Journal of Chemical Physics, 134(20).
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  PMID: 21639439;Abstract: We report photoelectron images and spectra of deprotonated thiophene, C4H3S-, obtained at 266, 355, and 390 nm. Photodetachment of the isomer of the anion is observed, and the photoelectron bands are assigned to the ground X2A′ (π) and excited A2A″ and B2A″ (π) states of the thiophenyl radical. The photoelectron angular distributions are consistent with photodetachment from the respective in-plane (Δ) and out-of-plane (π) orbitals. The adiabatic electron affinity of α-•C4H3S is determined to be 2.05 ± 0.08 eV, while the B2A″ term energy is estimated at 1.6 ± 0.1 eV. Using the measured electron affinity and the electron affinityacidity thermodynamic cycle, the C-Hα bond dissociation energy of thiophene is calculated as DH298(H α-C4H3S) = 115 ± 3 kcalmol. Comparison of this value to other, previously reported C-H bond dissociation energies, in particular for benzene and furan, sheds light of the relative thermodynamic stabilities of the corresponding radicals. In addition, the 266 nm photoelectron image and spectrum of the furanide anion, C4H 3O-, reveal a previously unobserved vibrationally resolved band, assigned to the B2A″ excited state of the furanyl radical, •C4H3O. The observed band origin corresponds to a 2.53 ± 0.01 eV B2A″ term energy, while the resolved vibrational progression (853 ± 42 cm -1) is assigned to an in-plane ring mode of α-•C4H3O (B2A″). © 2011 American Institute of Physics.
• Goebbert, D. J., Khuseynov, D., & Sanov, A. (2011). O^- + acetaldehyde reaction products: Search for singlet formylmethylene, a Wolff rearrangement intermediate. Journal of Physical Chemistry A, 115(15), 3208-3217.
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  PMID: 21438615;Abstract: The mass-resolved anionic products of the reaction of O•? with acetaldehyde, H3CCHO, are studied using photoelectron imaging. The primary anionic products are vinoxide, H2CCHO?, formylmethylene anion, HCCHO•?, and ketenylidene anion, CCO •?. From photoelectron spectra of HCCHO•?, the electron affinity of triplet (ground state) formylmethylene (1.87 ± 0.02 eV) and the vertical detachment energy corresponding to the first excited triplet state (3.05 eV) are determined, but no unambiguous assignment for singlet HCCHO could be made. The elusive singlet is a key intermediate in the Wolff rearrangement, resulting in formation of ketene. The fast rearrangement associated with a large geometry change upon photodetachment to the singlet surface may be responsible for the low intensity of the singlet compared to the triplet bands in the photoelectron spectrum. The title reaction also yields CCO•?, whose formation from acetaldehyde is novel and intriguing, since it requires a multistep net-H4+ abstraction. A possible mechanism is proposed, involving an [H 2CCO•?]* intermediate. From the measured electron affinities of HCCHO (above), H2CCHO (1.82 ± 0.01 eV), and CCO (2.31 ± 0.01 eV), several new thermochemical properties are determined, including the C?H bond dissociation energies and heats of formation of several organic molecules and/or their anions. Overall, the reactivity of O•? with organic molecules demonstrates the utility of this anion in the formation of a variety of reactive intermediates via a single process. © 2011 American Chemical Society.
• Grumbling, E. R., & Sanov, A. (2011). Photoelectron angular distributions in negative-ion photodetachment from mixed sp states. Journal of Chemical Physics, 135(16).
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  PMID: 22047234;Abstract: We describe an approach for constructing analytical models for the energy-dependence of photoelectron angular distributions in the one-electron, non-relativistic approximation. We construct such a model for electron emission from an orbital described as a superposition of s- and p-type functions, using linearly polarized light. In the limits of pure s or pure p electron photodetachment or photoionization, the model correctly reproduces the familiar Cooper-Zare formula. The model predictions are compared to experimental results for strongly solvated H- and NH2-, corresponding to predominantly s and predominantly p character parent states, respectively. © 2011 American Institute of Physics.
• Grumbling, E. R., & Sanov, A. (2011). Solvation effects on angular distributions in H^-(NH ₃)_n and NH₂^-(NH₃) _n photodetachment: Role of solute electronic structure. Journal of Chemical Physics, 135(16).
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  PMID: 22047233;Abstract: We report 355 and 532 nm photoelectron imaging results for H -(NH3)n and NH2-(NH 3)n, n=0-5. The photoelectron spectra are consistent with the electrostatic picture of a charged solute (H- or NH 2-) solvated by n ammonia molecules. For a given number of solvent molecules, the NH2- core anion is stabilized more strongly than H-, yet the photoelectron angular distributions for solvated H- deviate more strongly from the unsolvated limit than those for solvated NH2-. Hence, we conclude that solvation effects on photoelectron angular distributions are dependent on the electronic structure of the anion, i.e., the type of the initial orbital of the photodetached electron, rather than merely the strength of solvation interactions. We also find evidence of photofragmentation and autodetachment of NH2-(NH3)2-5, as well as autodetachment of H-(NH3)5, upon 532 nm excitation of these species. © 2011 American Institute of Physics.
• Grumbling, E. R., Pichugin, K., Mabbs, R., & Sanov, A. (2011). Photoelectron imaging as a quantum chemistry visualization tool. Journal of Chemical Education, 88(11), 1515-1520.
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  Abstract: An overview and simple example of photoelectron imaging is presented, highlighting its efficacy as a pedagogical tool for visualizing quantum phenomena. Specifically, photoelectron imaging of H- (the simplest negative ion) is used to demonstrate several quantum mechanical principles. This example could be incorporated into an introductory quantum chemistry course to extend the traditional discussion of the photoelectric effect and photoelectron spectroscopy into the area of matter waves. In working through this example, several core quantum-mechanical topics and concepts have been explored, such as conservation of angular momentum, the transition dipole moment, components of the hydrogenic orbitals, the Born interpretation of the wave function, and the theory of quantum measurement. © 2011 The American Chemical Society and Division of Chemical Education, Inc.
• Ichino, T., Villano, S. M., Gianola, A. J., Goebbert, D. J., Velarde, L., Sanov, A., Blanksby, S. J., Zhou, X., Hrovat, D. A., Borden, W. T., & Lineberger, W. C. (2011). Photoelectron spectroscopic study of the oxyallyl diradical. Journal of Physical Chemistry A, 115(9), 1634-1649.
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  PMID: 21323385;Abstract: The photoelectron spectrum of the oxyallyl (OXA) radical anion has been measured. The radical anion has been generated in the reaction of the atomic oxygen radical anion (O•-) with acetone. Three low-lying electronic states of OXA have been observed in the spectrum. Electronic structure calculations have been performed for the triplet states (3B2 and 3B1) of OXA and the ground doublet state ( 2A2) of the radical anion using density functional theory (DFT). Spectral simulations have been carried out for the triplet states based on the results of the DFT calculations. The simulation identifies a vibrational progression of the CCC bending mode of the 3B2 state of OXA in the lower electron binding energy (eBE) portion of the spectrum. On top of the 3B2 feature, however, the experimental spectrum exhibits additional photoelectron peaks whose angular distribution is distinct from that for the vibronic peaks of the 3B2 state. Complete active space self-consistent field (CASSCF) method and second-order perturbation theory based on the CASSCF wave function (CASPT2) have been employed to study the lowest singlet state (1A1) of OXA. The simulation based on the results of these electronic structure calculations establishes that the overlapping peaks represent the vibrational ground level of the 1A1 state and its vibrational progression of the CO stretching mode. The 1A1 state is the lowest electronic state of OXA, and the electron affinity (EA) of OXA is 1.940 ± 0.010 eV. The 3B2 state is the first excited state with an electronic term energy of 55 ± 2 meV. The widths of the vibronic peaks of the X̃ 1A1 state are much broader than those of the ã 3B2 state, implying that the 1A 1 state is indeed a transition state. The CASSCF and CASPT2 calculations suggest that the 1A1 state is at a potential maximum along the nuclear coordinate representing disrotatory motion of the two methylene groups, which leads to three-membered-ring formation, i.e., cyclopropanone. The simulation of b̃ 3B1 OXA reproduces the higher eBE portion of the spectrum very well. The term energy of the 3B1 state is 0.883 ± 0.012 eV. Photoelectron spectroscopic measurements have also been conducted for the other ion products of the O•- reaction with acetone. The photoelectron imaging spectrum of the acetylcarbene (AC) radical anion exhibits a broad, structureless feature, which is assigned to the X̃ 3A″ state of AC. The ground ( 2A″) and first excited (2A′) states of the 1-methylvinoxy (1-MVO) radical have been observed in the photoelectron spectrum of the 1-MVO ion, and their vibronic structure has been analyzed. © 2011 American Chemical Society.
• Sanov, A., Culberson, L. M., & Sanov, A. M. (2011). Electronic states of thiophenyl and furanyl radicals and dissociation energy of thiophene via photoelectron imaging of negative ions. The Journal of chemical physics, 134(20).
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  We report photoelectron images and spectra of deprotonated thiophene, C(4)H(3)S(-), obtained at 266, 355, and 390 nm. Photodetachment of the α isomer of the anion is observed, and the photoelectron bands are assigned to the ground X(2)A(') (σ) and excited A(2)A(") and B(2)A(") (π) states of the thiophenyl radical. The photoelectron angular distributions are consistent with photodetachment from the respective in-plane (σ) and out-of-plane (π(∗)) orbitals. The adiabatic electron affinity of α-(●)C(4)H(3)S is determined to be 2.05 ± 0.08 eV, while the B(2)A(") term energy is estimated at 1.6 ± 0.1 eV. Using the measured electron affinity and the electron affinity/acidity thermodynamic cycle, the C-H(α) bond dissociation energy of thiophene is calculated as DH(298)(H(α)-C(4)H(3)S) = 115 ± 3 kcal/mol. Comparison of this value to other, previously reported C-H bond dissociation energies, in particular for benzene and furan, sheds light of the relative thermodynamic stabilities of the corresponding radicals. In addition, the 266 nm photoelectron image and spectrum of the furanide anion, C(4)H(3)O(-), reveal a previously unobserved vibrationally resolved band, assigned to the B(2)A(") excited state of the furanyl radical, (●)C(4)H(3)O. The observed band origin corresponds to a 2.53 ± 0.01 eV B(2)A(") term energy, while the resolved vibrational progression (853 ± 42 cm(-1)) is assigned to an in-plane ring mode of α-(●)C(4)H(3)O (B(2)A(")).
• Sanov, A., Goebbert, D. J., Khuseynov, D., & Sanov, A. M. (2011). O(-) + acetaldehyde reaction products: search for singlet formylmethylene, a Wolff rearrangement intermediate. The journal of physical chemistry. A, 115(15).
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  The mass-resolved anionic products of the reaction of O(•-) with acetaldehyde, H(3)CCHO, are studied using photoelectron imaging. The primary anionic products are vinoxide, H(2)CCHO(-), formylmethylene anion, HCCHO(•-), and ketenylidene anion, CCO(•-). From photoelectron spectra of HCCHO(•-), the electron affinity of triplet (ground state) formylmethylene (1.87 ± 0.02 eV) and the vertical detachment energy corresponding to the first excited triplet state (3.05 eV) are determined, but no unambiguous assignment for singlet HCCHO could be made. The elusive singlet is a key intermediate in the Wolff rearrangement, resulting in formation of ketene. The fast rearrangement associated with a large geometry change upon photodetachment to the singlet surface may be responsible for the low intensity of the singlet compared to the triplet bands in the photoelectron spectrum. The title reaction also yields CCO(•-), whose formation from acetaldehyde is novel and intriguing, since it requires a multistep net-H(4)(+) abstraction. A possible mechanism is proposed, involving an [H(2)CCO(•-)]* intermediate. From the measured electron affinities of HCCHO (above), H(2)CCHO (1.82 ± 0.01 eV), and CCO (2.31 ± 0.01 eV), several new thermochemical properties are determined, including the C-H bond dissociation energies and heats of formation of several organic molecules and/or their anions. Overall, the reactivity of O(•-) with organic molecules demonstrates the utility of this anion in the formation of a variety of reactive intermediates via a single process.
• Sanov, A., Grumbling, E. R., & Sanov, A. M. (2011). Photoelectron angular distributions in negative-ion photodetachment from mixed sp states. The Journal of chemical physics, 135(16).
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  We describe an approach for constructing analytical models for the energy-dependence of photoelectron angular distributions in the one-electron, non-relativistic approximation. We construct such a model for electron emission from an orbital described as a superposition of s- and p-type functions, using linearly polarized light. In the limits of pure s or pure p electron photodetachment or photoionization, the model correctly reproduces the familiar Cooper-Zare formula. The model predictions are compared to experimental results for strongly solvated H(-) and NH(2)(-), corresponding to predominantly s and predominantly p character parent states, respectively.
• Sanov, A., Grumbling, E. R., & Sanov, A. M. (2011). Solvation effects on angular distributions in H- (NH3)n and NH2(-) (NH3)n photodetachment: role of solute electronic structure. The Journal of chemical physics, 135(16).
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  We report 355 and 532 nm photoelectron imaging results for H(-)(NH(3))(n) and NH(2)(-)(NH(3))(n), n = 0-5. The photoelectron spectra are consistent with the electrostatic picture of a charged solute (H(-) or NH(2)(-)) solvated by n ammonia molecules. For a given number of solvent molecules, the NH(2)(-) core anion is stabilized more strongly than H(-), yet the photoelectron angular distributions for solvated H(-) deviate more strongly from the unsolvated limit than those for solvated NH(2)(-). Hence, we conclude that solvation effects on photoelectron angular distributions are dependent on the electronic structure of the anion, i.e., the type of the initial orbital of the photodetached electron, rather than merely the strength of solvation interactions. We also find evidence of photofragmentation and autodetachment of NH(2)(-)(NH(3))(2-5), as well as autodetachment of H(-)(NH(3))(5), upon 532 nm excitation of these species.
• Duzor, M. V., Mbaiwa, F., Wei, J., Singh, T., Mabbs, R., Sanov, A., Cavanagh, S. J., Gibson, S. T., Lewis, B. R., & Gascooke, J. R. (2010). Vibronic coupling in the superoxide anion: The vibrational dependence of the photoelectron angular distribution. Journal of Chemical Physics, 133(17).
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  PMID: 21054036;Abstract: We present a comprehensive photoelectron imaging study of theO 2(X2∑g, v′ =0-6) ←O 2- (X2Πg, v″ =0) and O 2 (a 1Δg, v′=0-4)←O 2-((X2Πg, v″ =0) photodetachment bands at wavelengths between 900 and 455 nm, examining the effect of vibronic coupling on the photoelectron angular distribution (PAD). This work extends the v′ =1-4 data for detachment into the ground electronic state, presented in a recent communication [R. Mabbs, F. Mbaiwa, J. Wei, M. Van Duzor, S. T. Gibson, S. J. Cavanagh, and B. R. Lewis, Phys. Rev. A 82, 011401(R) (2010)]. Measured vibronic intensities are compared to Franck-Condon predictions and used as supporting evidence of vibronic coupling. The results are analyzed within the context of the one-electron, zero core contribution (ZCC) model [R. M. Stehman and S. B. Woo, Phys. Rev. A 23, 2866 (1981)]. For both bands, the photoelectron anisotropy parameter variation with electron kinetic energy, β (E), displays the characteristics of photodetachment from a d -like orbital, consistent with the πg 2p highest occupied molecular orbital ofO2-. However, differences exist between the β trends for detachment into different vibrational levels of the X 2∑g and a 1Δg electronic states ofO2-. The ZCC model invokes vibrational channel specific "detachment orbitals" and attributes this behavior to coupling of the electronic and nuclear motion in the parent anion. The spatial extent of the model detachment orbital is dependent on the final state of O2: the higher the neutral vibrational excitation, the larger the electron binding energy. Although vibronic coupling is ignored in most theoretical treatments of PADs in the direct photodetachment of molecular anions, the present findings clearly show that it can be important. These results represent a benchmark data set for a relatively simple system, upon which to base rigorous tests of more sophisticated models. © 2010 American Institute of Physics.
• Goebbert, D. J., Khuseynov, D., & Sanov, A. (2010). Photodissociation of nitromethane cluster anions. Journal of Chemical Physics, 133(8).
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  PMID: 20815573;Abstract: Three types of anionic fragments are observed in the photodissociation of nitromethane cluster anions, (CH3 NO2) n-, n=1-6, at 355 nm: NO2- (CH3 NO2) k, (CH 3 NO2)k-, and OH- (k
• Goebbert, D. J., Khuseynov, D., & Sanov, A. (2010). Photoelectron imaging of cyanovinylidene and cyanoacetylene anions. Journal of Physical Chemistry A, 114(6), 2259-2265.
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  PMID: 20092319;Abstract: Negative ions of cyanoacetylene and cyanovinylidene are generated simultaneously via the competing 1,1H2+ and 1,2-H 2+ abstraction channels of O- reaction with acrylonitrile. The two stable isomeric forms of the anion, CCHCN- and HCCCN-, are separated by a large (∼2 eV) potential energy barrier. Their photodetachment provides access to both the reactant and the product sides of the neutral cyanovinylidene -* cyanoacetylene rearrangement reaction, predicted to involve only a very small barrier. Using photoelectron imaging spectroscopy at 532 and 355 nm, the adiabatic electron affinity of the reactive intermediate :C=CHCN (X 1A'), is determined to be 1.84 ± 0.01 eV. The photoelectron spectrum of CCIICN-exhibits a vibrational progression attributed to the excitation of the CCII bending mode. The observed spectral features are reproduced reasonably well using a Franck-Condon simulation under the parallel-mode approximation. In contrast to unsubstituted acetylene, cyanoacetylene has a stable anionic state, which is adiabatically weakly bound, but has an experimentally determined vertical detachment energy of 1.04 ± 0.05 eV. This measurement, along with the broad, structureless photoelectron spectrum, of HCCCN-(with no identifiable origin), reflects the large geometry difference between the w-shaped structure of the anion and the linear equilibrium, geometry of HCCCN. © 2010 American Chemical Society.
• Goebbert, D. J., Pichugin, K., Khuseynov, D., Wenthold, P. G., & Sanov, A. (2010). Photoelectron imaging of NCCCN^-: The triplet ground state and the singlet-triplet splitting of dicyanocarbene. Journal of Chemical Physics, 132(22).
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  PMID: 20550391;Abstract: The photoelectron spectra of NCCCN- have been measured at 355 and 266 nm by means of photoelectron imaging. The spectra show two distinct features, corresponding to the ground and first excited states of dycianocarbene. With support from theoretical calculations using the spin-flip coupled-cluster methods, the ground electronic state of HCCCN is assigned as a triplet state, while the first excited state is a closed-shell singlet. The photoelectron band corresponding to the triplet is broad and congested, indicating a large geometry change between the anion and neutral. A single sharp feature of the singlet band suggests that the geometry of the excited neutral is similar to that of the anion. In agreement with these observations, theoretical calculations show that the neutral triplet state is either linear or quasilinear (X̃ 3B1 or 3∑-g), while the closed-shell singlet (ã 1A1) geometry is strongly bent, similar to the anion structure. The adiabatic electron binding energy of the closed-shell singlet is measured to be 3.72±0.02 eV. The best estimate of the origin of the triplet band gives an experimental upper bound of the adiabatic electron affinity of NCCCN, EA≤3.25±0.05 eV, while the Franck-Condon modeling yields an estimate of EA (NCCCN) =3.20±0.05 eV. From these results, the singlet-triplet splitting is estimated to be Δ EST (X̃3B1/3∑ -Bg-ã 1A1) =0.52±0.05 eV (12.0±1.2 kcal/mol). © 2010 American Institute of Physics.
• Goebbert, D. J., Velarde, L., Khuseynov, D., & Sanov, A. (2010). C-H bond dissociation energy of malononitrile. Journal of Physical Chemistry Letters, 1(4), 792-795.
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  Abstract: The C-H bond dissociation energies of closed-shell molecules decrease with increasing stability of the resulting radicals. From the electron affinity of the dicyanomethyl radical, •CH(CN)2, EA[•CH(CN) 2]=2.88± 0.01 eV, measured by photoelectron imaging of the CH(CN)2- anion, and the acidity/electron affinity thermodynamic cycle, we obtained the C-H bond dissociation enthalpy of malononitrile, CH2(CN)2, DH298[H-CH(CN) 2]=87 ± 2 kcal/mol. This result is compared to the corresponding value for acetonitrile, DH298(H-CH2CN)=93 ± 2 kcal/mol, determined from a similar measurement of EA(•CH2CN) = 1.53 ( 0.01 eV. The relative weakness of the C-H bonds in malononitrile and acetonitrile, compared to most closed-shell neutral organic molecules, is attributed to π-resonance stabilization of the unpaired electrons in •CH(CN)2 and •CH2CN. © 2010 American Chemical Society.
• Goebbert, D. J., Wende, T., Jiang, L., Meijer, G., Sanov, A., & Asmis, K. R. (2010). IR spectroscopic characterization of the thermally induced isomerization in carbon disulfide dimer anions. Journal of Physical Chemistry Letters, 1(16), 2465-2469.
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  Abstract: We report experimental vibrational spectra of thermalized carbon disulfide dimer anions, (CS2)2-, measured at ion trap temperatures from 16 to 300 K. Previous experiments showed evidence for several (CS2)2- isomers, whose relative abundance depends on the source conditions. We used infrared (IR) photodissociation spectroscopy in the fingerprint region (550-1600 cm-1) of (CS 2)2- thermalized in a temperature-controllable ion trap, in combination with simulated IR spectra derived from ab initio calculations, to identify the isomers present at various ion trap temperatures. The IR photodissociation spectra show characteristic signatures for at least three different isomers. Anions formed in the source are primarily trapped as high-energy ion-molecule complexes, in which the unpaired electron is localized on a single CS2 moiety. Thermal heating supplies sufficient energy to overcome the isomerization barriers and shifts the isomer population via a weakly bound isomer, in which the electron is delocalized over the complete complex, to lower-energy covalently bound structures. © 2010 American Chemical Society.
• Grumbling, E. R., Pichugin, K., Velarde, L., & Sanov, A. (2010). Further evidence for resonant photoelectron-solvent scattering in nitrous oxide cluster anions. Journal of Physical Chemistry A, 114(3), 1367-1373.
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  PMID: 19817360;Abstract: The effects of anion solvation by N2O on photoelectron angular distributions are revisited in light of new photoelectron imaging results for the NO-(N2O)n, n = 0-4 cluster anions at 266 nm. The new observations are examined in the context of the previous studies of O- and NO- anions solvated in the gas phase by nitrous oxide [Pichugin; et al. J. Chem. Phys. et al. 2008, 129, 044311.; Velarde; et al. J. Chem. Phys. et al. 2007, 127, 084302.]. The photoelectron angular distributions collected in the three separate studies are summarized and analyzed using bare O- and NO- as zero-solvation references. Solvent-induced deviations of the angular distributions from the zero-solvation reference are scaled by solvation number (n) to yield solvent-induced anisotropy differentials. These differentials, calculated identically for the O-(N2O)n and NO-(N 2O)n cluster series, show remarkably similar energy dependences, peaking in the vicinity of a known electron-N2O scattering resonance. The results support the conclusion that the solvation effect on the photoelectron angular distributions in these cases is primarily due to resonant interaction of photoelectrons with the N2O solvent, rather than a solvent-induced perturbation of the parent-anion electronic wave function. © 2010 American Chemical Society.
• Mozhayskiy, V., Goebbert, D. J., Velarde, L., Sanov, A., & Krylov, A. I. (2010). Electronic structure and spectroscopy of oxyallyl: A theoretical study. Journal of Physical Chemistry A, 114(26), 6935-6943.
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  PMID: 20550159;Abstract: Electronic structure of the oxyallyl diradical and the anion is investigated using high-level ab initio methods. Converged theoretical estimates of the energy differences between low-lying electronic states of oxyallyl (OXA) as well as detachment energies of the anion are reported. Our best estimates of the adiabatic energy differences between the anion 2A2 and the neutral 3B2 and 3B1 states are 1.94 and 2.73 eV, respectively. The 1A1 state lies above 3B2 vertically, but geometric relaxation brings it below the triplet. The two-dimensional scan of the singlet 1A 1 potential energy surface (PES) reveals that there is no minimum corresponding to a singlet diradical structure. Thus, singlet OXA undergoes prompt barrierless ring closure. However, a flat shape of the PES results in the resonance trapping in the Franck-Condon region, giving rise to the experimentally observable features in the photoelectron spectrum. By performing reduced-dimensionality wave packet calculations, we estimated that the wave packet lingers in the Franck-Condon region for about 170 fs, which corresponds to the spectral line broadening of about 200 cm-1. We also present calculations of the photodetachment spectrum and compare it with experimental data. Our calculations lend strong support to the assignment of the photoelectron spectrum of the OXA anion reported in Ichino et al. (Angew. Chem., Int. Ed. Engl. 2009, 48, 8509). © 2010 American Chemical Society.
• Sanov, A., Goebbert, D. J., Khuseynov, D., & Sanov, A. M. (2010). Photodissociation of nitromethane cluster anions. The Journal of chemical physics, 133(8).
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  Three types of anionic fragments are observed in the photodissociation of nitromethane cluster anions, (CH(3)NO(2))(n)(-), n=1-6, at 355 nm: NO(2)(-)(CH(3)NO(2))(k), (CH(3)NO(2))(k)(-), and OH(-) (k
• Sanov, A., Goebbert, D. J., Khuseynov, D., & Sanov, A. M. (2010). Photoelectron imaging of cyanovinylidene and cyanoacetylene anions. The journal of physical chemistry. A, 114(6).
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  Negative ions of cyanoacetylene and cyanovinylidene are generated simultaneously via the competing 1,1-H(2)(+) and 1,2-H(2)(+) abstraction channels of O(-) reaction with acrylonitrile. The two stable isomeric forms of the anion, CCHCN(-) and HCCCN(-), are separated by a large (approximately 2 eV) potential energy barrier. Their photodetachment provides access to both the reactant and the product sides of the neutral cyanovinylidene --> cyanoacetylene rearrangement reaction, predicted to involve only a very small barrier. Using photoelectron imaging spectroscopy at 532 and 355 nm, the adiabatic electron affinity of the reactive intermediate :C horizontal lineCHCN (X(1)A'), is determined to be 1.84 +/- 0.01 eV. The photoelectron spectrum of CCHCN(-) exhibits a vibrational progression attributed to the excitation of the CCH bending mode. The observed spectral features are reproduced reasonably well using a Franck-Condon simulation under the parallel-mode approximation. In contrast to unsubstituted acetylene, cyanoacetylene has a stable anionic state, which is adiabatically weakly bound, but has an experimentally determined vertical detachment energy of 1.04 +/- 0.05 eV. This measurement, along with the broad, structureless photoelectron spectrum of HCCCN(-) (with no identifiable origin), reflects the large geometry difference between the w-shaped structure of the anion and the linear equilibrium geometry of HCCCN.
• Sanov, A., Goebbert, D. J., Pichugin, K., Khuseynov, D., Wenthold, P. G., & Sanov, A. M. (2010). Photoelectron imaging of NCCCN(-): The triplet ground state and the singlet-triplet splitting of dicyanocarbene. The Journal of chemical physics, 132(22).
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  The photoelectron spectra of NCCCN(-) have been measured at 355 and 266 nm by means of photoelectron imaging. The spectra show two distinct features, corresponding to the ground and first excited states of dycianocarbene. With support from theoretical calculations using the spin-flip coupled-cluster methods, the ground electronic state of HCCCN is assigned as a triplet state, while the first excited state is a closed-shell singlet. The photoelectron band corresponding to the triplet is broad and congested, indicating a large geometry change between the anion and neutral. A single sharp feature of the singlet band suggests that the geometry of the excited neutral is similar to that of the anion. In agreement with these observations, theoretical calculations show that the neutral triplet state is either linear or quasilinear (X (3)B(1) or (3)Sigma(g) (-)), while the closed-shell singlet (a (1)A(1)) geometry is strongly bent, similar to the anion structure. The adiabatic electron binding energy of the closed-shell singlet is measured to be 3.72+/-0.02 eV. The best estimate of the origin of the triplet band gives an experimental upper bound of the adiabatic electron affinity of NCCCN, EA
• Sanov, A., Grumbling, E. R., Pichugin, K., Velarde, L., & Sanov, A. M. (2010). Further evidence for resonant photoelectron-solvent scattering in nitrous oxide cluster anions. The journal of physical chemistry. A, 114(3).
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  The effects of anion solvation by N(2)O on photoelectron angular distributions are revisited in light of new photoelectron imaging results for the NO(-)(N(2)O)(n), n = 0-4 cluster anions at 266 nm. The new observations are examined in the context of the previous studies of O(-) and NO(-) anions solvated in the gas phase by nitrous oxide [Pichugin; et al. J. Chem. Phys. et al. 2008, 129, 044311.; Velarde; et al. J. Chem. Phys. et al. 2007, 127, 084302.]. The photoelectron angular distributions collected in the three separate studies are summarized and analyzed using bare O(-) and NO(-) as zero-solvation references. Solvent-induced deviations of the angular distributions from the zero-solvation reference are scaled by solvation number (n) to yield solvent-induced anisotropy differentials. These differentials, calculated identically for the O(-)(N(2)O)(n) and NO(-)(N(2)O)(n) cluster series, show remarkably similar energy dependences, peaking in the vicinity of a known electron-N(2)O scattering resonance. The results support the conclusion that the solvation effect on the photoelectron angular distributions in these cases is primarily due to resonant interaction of photoelectrons with the N(2)O solvent, rather than a solvent-induced perturbation of the parent-anion electronic wave function.
• Goebbert, D. J., & Sanov, A. (2009). Photodetachment, photofragmentation, and fragment autodetachment of [O _2n(H₂ O) _m] ^- Clusters: Core-anion structures and fragment energy partitioning. Journal of Chemical Physics, 131(10).
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  Abstract: Building on the past studies of the O 2n- and O 2- (H2 O) m cluster anion series, we assess the effect of the strong hydration interactions on the oxygen-core clusters using photoelectron imaging and photofragment mass spectroscopy of [O2n (H2 O) m]- (n=1-4, m=0-3) at 355 nm. The results show that both pure-oxygen and hydrated clusters with n≥2 form an O4- core anion, indicated in the past work on the pure-oxygen clusters. All clusters studied can be therefore described in terms of O4- (H2 O) m (O2) n-2 structures, although the O4- core may be strongly perturbed by hydration in some of these clusters. Fragmentation of these clusters yields predominantly O2- and O 2- (H2 O) l (l
• Goebbert, D. J., Khuseynov, D., & Sanov, A. (2009). Laboratory observation of the valence anion of cyanoacetylene, a possible precursor for negative ions in space. Journal of Chemical Physics, 131(16).
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  PMID: 19894914;Abstract: Valence anions of cyanoacetylene, HCCCN-, are synthesized by the 1,2 -H2+ abstraction reaction of O- with acrylonitrile, H2 C=CHCN, while the competing 1,1 -H2+ channel of the same reaction yields the cyanovinylidene anions, CCHCN-. The key to the formation of the elusive, adiabatically weakly bound HCCCN- is the bent -̇C = ̇C-Cskeleton of the reactant. The photoelectron spectrum of HCCCN -, measured by means of photoelectron imaging at 532 nm, consists of a broad structureless band with a vertical detachment energy of 1.04±0.05 eV. The observed anions are stable counterparts of the low-lying anionic resonances of cyanoacetylene, which may contribute (by way of dissociative attachment) to the formation of carbon-rich and CN-containing negative ions in extraterrestrial environments. © 2009 American Institute of Physics.
• Goebbert, D. J., Pichugin, K., & Sanov, A. (2009). Low-lying electronic states of CH₃NO₂ via photoelectron imaging of the nitromethane anion. Journal of Chemical Physics, 131(16).
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  PMID: 19894948;Abstract: Negative-ion photoelectron imaging at 532, 392, 355, and 266 nm is used to assign several low-lying electronic states of neutral nitromethane CH 3NO2 at the geometry corresponding to the anion equilibrium. The observed neutral states include (in the order of increasing binding energy) the X A1 ′ ground state, two triplet excited states, a 3A″ and b 3A″, and the first excited singlet state, A 1A″. The state assignments are aided by the analysis of the photoelectron angular distributions resulting from electron detachment from the a′ and a″ symmetry molecular orbitals and the results of theoretical calculations. The singlet-triplet (X 1A′ -a 3A″) splitting in nitromethane is determined as 2.90 +0.02/-0.07 eV, while the vibrational structure of the band corresponding to the formation of the a 3A″ state of CH3NO2 is attributed to the ONO bending and NO2 wagging motions excited in the photodetachment of the anion. © 2009 American Institute of Physics.
• Habteyes, T., Velarde, L., & Sanov, A. (2009). Effects of isomer coexistence and solvent-induced core switching in the photodissociation of bare and solvated (CS₂)₂^- anions. Journal of Chemical Physics, 130(12).
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  PMID: 19334824;Abstract: The photodissociation of the (CS2)2- dimer anion, known to exist in the form of several electronic and structural isomers, has been investigated at 532, 355, and 266 nm. The observed anionic fragments are CS 2- and C2 S2- at 532 nm, and C 2 S2-, CS 2-, CS 3-, S2-, and S- at 355 and 266 nm. In addition to the photon energy, the fractional yields of the photofragments depend on the ion source conditions and solvation of the dimer anion. Specifically, the (C2 S2- + S2-) / CS 2- product ratio is significantly higher when (CS2)2- is formed in the presence of water in the precursor gas mixture, even though the parent anion itself does not include H2 O. On the other hand, an abrupt decrease in the above product ratio is observed upon the addition of solvent molecules (CS2 or H2 O) to the (CS2)2- anion. Since the variation of this product ratio exhibits positive correlation with the relative intensity of the photoelectron band assigned to the C2v (B2 1) covalent structure of C2 S4- by Habteyes [J. Phys. Chem. A 112, 10134 (2008)], this structure is suggested as the primary origin of the C2 S 2- and S2- photoproducts. The switching of the fragmentation yield from C2 S2- and S2- to other products upon solvation is ascribed to the diminished presence of the C2v (B2 1) dimer-anion structure relative to the CS 2- based clusters. This population shift is attributed to the more effective solvation of the latter. The CS 2- based clusters are suggested as the origin of the S- photoproduct, while CS 3- is formed through the secondary S- + CS2 intracluster association reaction. © 2009 American Institute of Physics.
• Ichino, T., Villano, S. M., Gianola, A. J., Goebbert, D. J., Sanov, A., Velarde, L., Blanksby, S. J., Zhou, X., Hrovat, D. A., Borden, W. T., & Lineberger, W. C. (2009). The lowest singlet and triplet states of the oxyallyl diradical. Angewandte Chemie - International Edition, 48(45), 8509-8511.
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  PMID: 19739188;Abstract: Small S-T splitting: The photoelectron spectrum of the oxyallyl radical anion (see picture) reveals that the electronic ground state of oxyallyl is singlet, and the lowest triplet state is separated from the singlet state by only (55 ± 2) meV in adiabatic energy. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.
• Mabbs, R., Grumbling, E. R., Pichugin, K., & Sanov, A. (2009). Photoelectron imaging: An experimental window into electronic structure. Chemical Society Reviews, 38(8), 2169-2177.
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  PMID: 19623341;Abstract: Photoelectron imaging is finding increasingly widespread use in probing electronic structure and chemical dynamics. In this tutorial review, two benchmark systems, H- and I-, are used to introduce essential concepts linking photoelectron images of negative ions with parent electronic structure. For pedagogical reasons, a qualitative approach based upon spectroscopic selection rules is emphasized in interpreting the images. This approach is extended to molecular systems, highlighting that even qualitative interpretation of results can lead to significant chemical insights. © 2009 The Royal Society of Chemistry.
• Sanov, A., Goebbert, D. J., Khuseynov, D., & Sanov, A. M. (2009). Laboratory observation of the valence anion of cyanoacetylene, a possible precursor for negative ions in space. The Journal of chemical physics, 131(16).
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  Valence anions of cyanoacetylene, HCCCN(-), are synthesized by the 1,2-H(2) (+) abstraction reaction of O(-) with acrylonitrile, H(2)C=CHCN, while the competing 1,1-H(2) (+) channel of the same reaction yields the cyanovinylidene anions, CCHCN(-). The key to the formation of the elusive, adiabatically weakly bound HCCCN(-) is the bent -C=C-C[triple bond]skeleton of the reactant. The photoelectron spectrum of HCCCN(-), measured by means of photoelectron imaging at 532 nm, consists of a broad structureless band with a vertical detachment energy of 1.04+/-0.05 eV. The observed anions are stable counterparts of the low-lying anionic resonances of cyanoacetylene, which may contribute (by way of dissociative attachment) to the formation of carbon-rich and CN-containing negative ions in extraterrestrial environments.
• Sanov, A., Goebbert, D. J., Pichugin, K., & Sanov, A. M. (2009). Low-lying electronic states of CH(3)NO(2) via photoelectron imaging of the nitromethane anion. The Journal of chemical physics, 131(16).
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  Negative-ion photoelectron imaging at 532, 392, 355, and 266 nm is used to assign several low-lying electronic states of neutral nitromethane CH(3)NO(2) at the geometry corresponding to the anion equilibrium. The observed neutral states include (in the order of increasing binding energy) the X (1)A(') ground state, two triplet excited states, a (3)A(") and b (3)A("), and the first excited singlet state, A (1)A("). The state assignments are aided by the analysis of the photoelectron angular distributions resulting from electron detachment from the a(') and a(") symmetry molecular orbitals and the results of theoretical calculations. The singlet-triplet (X (1)A(')-a (3)A(")) splitting in nitromethane is determined as 2.90(+0.02)/(-0.07) eV, while the vibrational structure of the band corresponding to the formation of the a (3)A(") state of CH(3)NO(2) is attributed to the ONO bending and NO(2) wagging motions excited in the photodetachment of the anion.
• Sanov, A., Habteyes, T., Velarde, L., & Sanov, A. M. (2009). Effects of isomer coexistence and solvent-induced core switching in the photodissociation of bare and solvated (CS2)2(-) anions. The Journal of chemical physics, 130(12).
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  The photodissociation of the (CS(2))(2)(-) dimer anion, known to exist in the form of several electronic and structural isomers, has been investigated at 532, 355, and 266 nm. The observed anionic fragments are CS(2)(-) and C(2)S(2)(-) at 532 nm, and C(2)S(2)(-), CS(2)(-), CS(3)(-), S(2)(-), and S(-) at 355 and 266 nm. In addition to the photon energy, the fractional yields of the photofragments depend on the ion source conditions and solvation of the dimer anion. Specifically, the (C(2)S(2)(-) + S(2)(-))/CS(2)(-) product ratio is significantly higher when (CS(2))(2)(-) is formed in the presence of water in the precursor gas mixture, even though the parent anion itself does not include H(2)O. On the other hand, an abrupt decrease in the above product ratio is observed upon the addition of solvent molecules (CS(2) or H(2)O) to the (CS(2))(2)(-) anion. Since the variation of this product ratio exhibits positive correlation with the relative intensity of the photoelectron band assigned to the C(2v) ((2)B(1)) covalent structure of C(2)S(4)(-) by Habteyes et al. [J. Phys. Chem. A 112, 10134 (2008)], this structure is suggested as the primary origin of the C(2)S(2)(-) and S(2)(-) photoproducts. The switching of the fragmentation yield from C(2)S(2)(-) and S(2)(-) to other products upon solvation is ascribed to the diminished presence of the C(2v) ((2)B(1)) dimer-anion structure relative to the CS(2)(-) based clusters. This population shift is attributed to the more effective solvation of the latter. The CS(2)(-) based clusters are suggested as the origin of the S(-) photoproduct, while CS(3)(-) is formed through the secondary S(-) + CS(2) intracluster association reaction.
• Sanov, A., Velarde, L., Habteyes, T., Glass, R. S., & Sanov, A. M. (2009). Observation and characterization of the CH3S(O)CH- and CH3S(O)CH- x H2O carbene anions by photoelectron imaging and photofragment spectroscopy. The journal of physical chemistry. A, 113(15).
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  We report the observation of the CH(3)S(O)CH(-) and CH(3)S(O)CH(-) x H(2)O carbene anions formed upon overall H(2)(+) abstraction from dimethyl sulfoxide by O(-). Photoelectron spectroscopy reveals singlet and triplet carbenes for the remaining neutral, with the singlet state assigned as the ground state. Although some formation of the distonic CH(2)S(O)CH(2)(-) radical anion is also expected, no conclusive evidence of the presence of this isomer is found. The photoelectron spectrum of HCSO(-) is also reported for the first time. Photofragmentation of CH(3)S(O)CH(-) with 532 nm light reveals two main types of anionic products: a dominant HCSO(-) fragment, resulting from methyl elimination, and a less intense SO(-) product. For the monohydrated anion, an additional SO(-) x H(2)O fragment is observed. Intriguingly, both the SO(-) x H(2)O and SO(-) products are produced with much higher yields in the fragmentation of CH(3)S(O)CH(-) x H(2)O, compared to the SO(-) yield from the dissociation of the bare CH(3)S(O)CH(-) anion. Two possible pathways are proposed as likely mechanisms for the SO(-)-based photoproducts, both involving a photoinduced intramolecular rearrangement and the formation of a C-C bond.
• Habteyes, T., & Sanov, A. (2008). Electron binding motifs in the (CS₂)_n^- (n>4) cluster anions. Journal of Chemical Physics, 129(24).
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  PMID: 19123509;Abstract: Photoelectron imaging spectroscopy of (CS2)n-, n>4, reveals a new state with an electron binding energy smaller than that of any of the corresponding CS2- and C2S4- states known to date. With support from ab initio calculations, two long-lived, metastable binding motifs with small electron binding energies are discussed for these clusters. The first is a solvent network permeating state, where the excess electron is delocalized over a number of linear CS2 molecules. The second is an excited 2B1 state of the core CS2- anion stabilized at a slightly bent geometry by the solvation interactions. Based on the observed solvation-induced shifts in binding energy, the second motif is favored. © 2008 American Institute of Physics.
• Habteyes, T., Velarde, L., & Sanov, A. (2008). Relaxation of (CS₂)₂^- to its global minimum mediated by water molecules: Photoelectron imaging study. Journal of Physical Chemistry A, 112(41), 10134-10140.
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  PMID: 18808087;Abstract: The coexistence of several isomers of (CS2)2- is examined via photoelectron imaging at 355 and 266 nm. Assisted by theoretical calculations, the bands in the photoelectron spectra are assigned to the CS2-·CS2 ion-molecule complex (Cs symmetry, 2A′ electronic state) and two covalently bound dimer-anion structures: C2v (2B 1) and D2h (2B3g). The isomer distribution depends sensitively on the ion source conditions, particularly the presence of water in the precursor gas mixture. The intensity variation of the photoelectron bands suggests that the presence of water enhances the formation of the global-minimum C2v (2B1) structure, particularly relative to the metastable (local-minimum) ion-molecule complex. This trend is rationalized with two assumptions. The first is that the presence of H2O at the cluster formation stage facilitates the nonadiabatic transitions necessary for reaching the global-minimum dimer-anion equilibrium when starting from the CS2- + CS2 asymptote. The second is that the initial clusters formed in the presence of water tend to have, on average, more internal energy, which is needed for overcoming the potential energy barriers separating the metastable equilibria from the global-minimum structure. As the covalent bonds are formed, excess solvent molecules are evaporated from the cluster, giving rise to stable (CS 2)2- dimer anions. In the (CS2) n-, n > 3, and (CS2)2-(H2O)m, m > 0, clusters, the population of the covalent-dimer core structures diminishes drastically due to more favorable solvent interactions with the monomer-anion (i.e., CCS2-) core. © 2008 American Chemical Society.
• Pichugin, K., Grumbling, E., Velarde, L., & Sanov, A. (2008). Solvation-induced cluster anion core switching from NNO 2- (N2 O) n-1 to O- (N2 O) n. Journal of Chemical Physics, 129(4).
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  PMID: 18681650;Abstract: We report a photoelectron imaging study of the [O (N2 O) n] -, 0n9, cluster anions generated via electron bombardment of a pulsed supersonic expansion of pure N2 O gas. Depending on cluster size, the photoelectron image features and spectral trends, examined at 355 and 266 nm, give evidence of two dominant core-anion structures, corresponding to the NNO 2- (N2 O) n-1 and O- (N2 O) n cluster anions. In agreement with previous studies, the n=1 anion has a covalently bound (Y -shaped) NNO 2- structure. The NNO 2- core is also found to persist in the larger clusters, up to n=3. However, for n4 (and up to at least n=9) signatures of an O- core are predominantly observed. Photofragmentation studies at 355 nm support these results. © 2008 American Institute of Physics.
• Sanov, A., & Mabbs, R. (2008). Photoelectron imaging of negative ions. International Reviews in Physical Chemistry, 27(1), 53-85.
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  Abstract: This article provides an overview of some of the recent advances in the rapidly growing field of negative-ion photoelectron imaging spectroscopy. Setting the work that spans several projects in the authors' laboratory in broader context, three types of measurements are described. First are the 'static' (one-photon) photoelectron imaging experiments that aim to characterize the electronic structure and photodetachment dynamics of negative ions, providing 'signatures' of the bound electron orbitals. The experimental results are presented alongside a conceptual symmetry-based description of the photodetachment processes, enabling a qualitative interpretation of the photoelectron images. Second, the effects of solvation on the electronic structure and photodetachment dynamics are examined using photoelectron imaging of cluster anions. Third, the time-resolved experiments that target imaging of bond dissociation, as viewed from the electronic perspective along the time-resolved reaction coordinate, are described in the context of dual centre interference in molecular-anion photodetachment. © 2008 Taylor & Francis.
• Sanov, A., Habteyes, T., & Sanov, A. M. (2008). Electron binding motifs in the (CS2)n- (n>4) cluster anions. The Journal of chemical physics, 129(24).
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  Photoelectron imaging spectroscopy of (CS(2))(n) (-), n>4, reveals a new state with an electron binding energy smaller than that of any of the corresponding CS(2) (-) and C(2)S(4) (-) states known to date. With support from ab initio calculations, two long-lived, metastable binding motifs with small electron binding energies are discussed for these clusters. The first is a solvent network permeating state, where the excess electron is delocalized over a number of linear CS(2) molecules. The second is an excited (2)B(1) state of the core CS(2) (-) anion stabilized at a slightly bent geometry by the solvation interactions. Based on the observed solvation-induced shifts in binding energy, the second motif is favored.
• Sanov, A., Habteyes, T., Velarde, L., & Sanov, A. M. (2008). Relaxation of (CS2)2(-) to its global minimum mediated by water molecules: photoelectron imaging study. The journal of physical chemistry. A, 112(41).
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  The coexistence of several isomers of (CS 2) 2 (-) is examined via photoelectron imaging at 355 and 266 nm. Assisted by theoretical calculations, the bands in the photoelectron spectra are assigned to the CS 2 (-).CS 2 ion-molecule complex ( C s symmetry, (2)A' electronic state) and two covalently bound dimer-anion structures: C 2 v ( (2)B 1) and D 2 h ( (2)B 3g). The isomer distribution depends sensitively on the ion source conditions, particularly the presence of water in the precursor gas mixture. The intensity variation of the photoelectron bands suggests that the presence of water enhances the formation of the global-minimum C 2 v ( (2)B 1) structure, particularly relative to the metastable (local-minimum) ion-molecule complex. This trend is rationalized with two assumptions. The first is that the presence of H 2O at the cluster formation stage facilitates the nonadiabatic transitions necessary for reaching the global-minimum dimer-anion equilibrium when starting from the CS 2 (-) + CS 2 asymptote. The second is that the initial clusters formed in the presence of water tend to have, on average, more internal energy, which is needed for overcoming the potential energy barriers separating the metastable equilibria from the global-minimum structure. As the covalent bonds are formed, excess solvent molecules are evaporated from the cluster, giving rise to stable (CS 2) 2 (-) dimer anions. In the (CS 2) n (-), n >or= 3, and (CS 2) 2 (-)(H 2O) m , m > 0, clusters, the population of the covalent-dimer core structures diminishes drastically due to more favorable solvent interactions with the monomer-anion (i.e., CS 2 (-)) core.
• Sanov, A., Pichugin, K., Grumbling, E., Velarde, L., & Sanov, A. M. (2008). Solvation-induced cluster anion core switching from NNO2(-)(N2O)n-1 to O(-)(N2O)n. The Journal of chemical physics, 129(4).
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  We report a photoelectron imaging study of the [O(N(2)O)(n)](-), 0
• Habteyes, T., Velarde, L., & Sanov, A. (2007). Photodissociation of CO₂^- in water clusters via Renner-Teller and conical interactions. Journal of Chemical Physics, 126(15).
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  Abstract: The photochemistry of mass selected CO2-(H 2O)m, m=2-40 cluster anions is investigated using 266 nm photofragment spectroscopy and theoretical calculations. Similar to the previous 355 nm experiment [Habteyes et al., Chem. Phys. Lett. 424, 268 (2006)], the fragmentation at 266 nm yields two types of anionic products: O -(H2O)m-k (core-dissociation products) and CO2-(H2O)m-k (solvent-evaporation products). Despite the same product types, different electronic transitions and dissociation mechanisms are implicated at 355 and 266 nm. The 355 nm dissociation is initiated by excitation to the first excited electronic state of the CO2- cluster core, the 1 2B 1(2A″) state, and proceeds via a glancing Renner-Teller intersection with the ground electronic state at a linear geometry. The 266 nm dissociation involves the second excited electronic state of CO2-, the 2 2A1( 2A′) state, which exhibits a conical intersection with the 3 2B2(A′) state at a bent geometry. The asymptotic O- based products are believed to be formed via this 3 2B2(A′) state. By analyzing the fragmentation results, the bond dissociation energy of CO2- to O -+CO in hydrated clusters (m ≥ 20) is estimated as 2.49 eV, compared to 3.46 eV for bare CO2-. The enthalpy of evaporation of one water molecule from asymptotically large CO2-(H2O)m clusters is determined to be 0.466±0.001 eV (45.0±0.1 kJ/mol). This result compares very favorably with the heat of evaporation of bulk water, 0.456 eV (43.98 kJ/mol). © 2007 American Institute of Physics.
• Sanov, A. (2007). Chemistry: Coherence and symmetry breaking at the molecular level. Science, 315(5812), 610-611.
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  PMID: 17272711;Abstract: Contrary to expectation, when an electron leaves a symmetric dihydrogen molecule before dissociation, the breakup is asymmetric. This paradox can be understood by considering fundamental properties of interacting waves.
• Sanov, A., & Sanov, A. M. (2007). Chemistry. Coherence and symmetry breaking at the molecular level. Science (New York, N.Y.), 315(5812).
• Sanov, A., Habteyes, T., Velarde, L., & Sanov, A. M. (2007). Photodissociation of CO2 - in water clusters via Renner-Teller and conical interactions. The Journal of chemical physics, 126(15).
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  The photochemistry of mass selected CO(2) (-)(H2O)(m), m=2-40 cluster anions is investigated using 266 nm photofragment spectroscopy and theoretical calculations. Similar to the previous 355 nm experiment [Habteyes et al., Chem. Phys. Lett. 424, 268 (2006)], the fragmentation at 266 nm yields two types of anionic products: O(-)(H2O)(m-k) (core-dissociation products) and CO(2) (-)(H2O)(m-k) (solvent-evaporation products). Despite the same product types, different electronic transitions and dissociation mechanisms are implicated at 355 and 266 nm. The 355 nm dissociation is initiated by excitation to the first excited electronic state of the CO(2) (-) cluster core, the 1 (2)B(1)(2A") state, and proceeds via a glancing Renner-Teller intersection with the ground electronic state at a linear geometry. The 266 nm dissociation involves the second excited electronic state of CO(2) (-), the 2 (2)A(1)(2A') state, which exhibits a conical intersection with the 3 (2)B(2)(A') state at a bent geometry. The asymptotic O(-) based products are believed to be formed via this 3 (2)B(2)(A') state. By analyzing the fragmentation results, the bond dissociation energy of CO(2) (-) to O(-)+CO in hydrated clusters (m> or =20) is estimated as 2.49 eV, compared to 3.46 eV for bare CO(2) (-). The enthalpy of evaporation of one water molecule from asymptotically large CO(2) (-)(H(2)O)(m) clusters is determined to be 0.466+/-0.001 eV (45.0+/-0.1 kJ/mol). This result compares very favorably with the heat of evaporation of bulk water, 0.456 eV (43.98 kJ/mol).
• Sanov, A., Velarde, L., Habteyes, T., Grumbling, E. R., Pichugin, K., & Sanov, A. M. (2007). Solvent resonance effect on the anisotropy of NO(-)(N(2)O)(n) cluster anion photodetachment. The Journal of chemical physics, 127(8).
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  Photodetachment from NO(-)(N(2)O)(n) cluster anions (n< or =7) is investigated using photoelectron imaging at 786, 532, and 355 nm. Compared to unsolvated NO(-), the photoelectron anisotropy with respect to the laser polarization direction diminishes drastically in the presence of the N(2)O solvent, especially in the 355 nm data. In contrast, a less significant anisotropy loss is observed for NO(-)(H(2)O)(n). The effect is attributed to photoelectron scattering on the solvent, which in the N(2)O case is mediated by the (2)Pi anionic resonance. No anionic resonances exist for H(2)O in the applicable photoelectron energy range, in line with the observed difference between the photoelectron images obtained with the two solvents. The momentum-transfer cross section, rather than the total scattering cross section, is argued to be an appropriate physical parameter predicting the solvent effects on the photoelectron angular distributions in these cluster anions.
• Velarde, L., Habteyes, T., Grumbling, E. R., Pichugin, K., & Sanov, A. (2007). Solvent resonance effect on the anisotropy of N O- (N2 O)n cluster anion photodetachment. Journal of Chemical Physics, 127(8).
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  Abstract: Photodetachment from N O- (N2 O)n cluster anions (n≤7) is investigated using photoelectron imaging at 786, 532, and 355 nm. Compared to unsolvated N O-, the photoelectron anisotropy with respect to the laser polarization direction diminishes drastically in the presence of the N2 O solvent, especially in the 355 nm data. In contrast, a less significant anisotropy loss is observed for N O- (H2 O)n. The effect is attributed to photoelectron scattering on the solvent, which in the N2 O case is mediated by the Π2 anionic resonance. No anionic resonances exist for H2 O in the applicable photoelectron energy range, in line with the observed difference between the photoelectron images obtained with the two solvents. The momentum-transfer cross section, rather than the total scattering cross section, is argued to be an appropriate physical parameter predicting the solvent effects on the photoelectron angular distributions in these cluster anions. © 2007 American Institute of Physics.
• Akin, F. A., Schirra, L. K., & Sanov, A. (2006). Photoelectron imaging study of the effect of monohydration on O ₂^- photodetachment. Journal of Physical Chemistry A, 110(26), 8031-8036.
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  PMID: 16805488;Abstract: The photodetachment of the O 2-·H 2O cluster anion at 780 and 390 nm is investigated in comparison with O 2- using photoelectron imaging spectroscopy. Despite the pronounced shift in the photoelectron spectra, the monohydration has little effect on the photoelectron angular distributions: for a given wavelength and electron kinetic energy (eKE) range, the O 2-·H 2O angular distributions are quantitatively similar to those for bare O 2-. This observation confirms that the excess electron in O 2-·H 2O retains the overall character of the 2pπ g* HOMO of O 2-. The presence of H 2O does not affect significantly the partial wave composition of the photodetached electrons at a given eKE. An exception is observed for slow electrons, where O 2-·H 2O exhibits a faster rise in the photodetachment signal with increasing eKE, as compared to O 2-. The possible causes of this anomaly are (i) the long-range charge-dipole interaction between the departing electron and the neutral O 2·H 2O skeleton affecting the slow-electron dynamics; and (ii) the s wave contributions to the photodetachment, which are dipole-forbidden for π g-1 transitions in O 2-, but formally allowed in O 2-·H 2O due to lower symmetry of the cluster anion and the corresponding HOMO. © 2006 American Chemical Society.
• Habteyes, T., Velarde, L., & Sanov, A. (2006). Solvent-enabled photodissociation of CO₂^- in water clusters. Chemical Physics Letters, 424(4-6), 268-272.
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  Abstract: The photofragmentation of CO2- (H2 O)m, m = 3-20 at 355 nm yields two types of anionic products: O-(H2O)m-k, 1 ≤ k ≤ 3, and CO2- (H2 O)m - k, 4 ≤ k ≤ 9, depending on the parent cluster size. The O-(H2O)m-k fragments, attributed to the dissociation of hydrated CO2-, are dominant for m = 3-7, while the water-evaporation products, CO2- (H2 O)m - k, take precedence for m = 8-20. The dissociation of CO2- is proposed to proceed via a hydration-stabilized excited state, originating from a low-lying CO2- resonance. In the evaporation channel, the suggested routes include cluster predissociation, CO2- photodissociation/recombination, and charge transfer to solvent. © 2006 Elsevier B.V. All rights reserved.
• Sanov, A., Akin, F. A., Schirra, L. K., & Sanov, A. M. (2006). Photoelectron imaging study of the effect of monohydration on O2 - photodetachment. The journal of physical chemistry. A, 110(26).
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  The photodetachment of the O(2)(-).H(2)O cluster anion at 780 and 390 nm is investigated in comparison with O(2)(-) using photoelectron imaging spectroscopy. Despite the pronounced shift in the photoelectron spectra, the monohydration has little effect on the photoelectron angular distributions: for a given wavelength and electron kinetic energy (eKE) range, the O(2)(-).H(2)O angular distributions are quantitatively similar to those for bare O(2)(-). This observation confirms that the excess electron in O(2)(-).H(2)O retains the overall character of the 2ppi(g) HOMO of O(2)(-). The presence of H(2)O does not affect significantly the partial wave composition of the photodetached electrons at a given eKE. An exception is observed for slow electrons, where O(2)(-).H(2)O exhibits a faster rise in the photodetachment signal with increasing eKE, as compared to O(2)(-). The possible causes of this anomaly are (i) the long-range charge-dipole interaction between the departing electron and the neutral O(2).H(2)O skeleton affecting the slow-electron dynamics; and (ii) the s wave contributions to the photodetachment, which are dipole-forbidden for pi(g)(-1) transitions in O(2)(-), but formally allowed in O(2)(-).H(2)O due to lower symmetry of the cluster anion and the corresponding HOMO.
• Sanov, A., Velarde, L., Habteyes, T., & Sanov, A. M. (2006). Photodetachment and photofragmentation pathways in the [(CO2)2(H2O)m]- cluster anions. The Journal of chemical physics, 125(11).
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  The mass-selected [(CO(2))(2)(H(2)O)(m)](-) cluster anions are studied using a combination of photoelectron imaging and photofragment mass spectroscopy at 355 nm. Photoelectron imaging studies are carried out on the mass-selected parent cluster anions in the m=2-6 size range; photofragmentation results are presented for m=3-11. While the photoelectron images suggest possible coexistence of the CO(2) (-)(H(2)O)(m)CO(2) and (O(2)CCO(2))(-)(H(2)O)(m) parent cluster structures, particularly for m=2 and 3, only the CO(2) (-) based clusters are both required and sufficient to explain all fragmentation pathways for m>/=3. Three types of anionic photofragments are observed: CO(2) (-)(H(2)O)(k), O(-)(H(2)O)(k), and CO(3) (-)(H(2)O)(k), k
• Velarde, L., Habteyes, T., & Sanov, A. (2006). Photodetachment and photofragmentation pathways in the [(CO ₂) ₂(H ₂O) _m] ^- cluster anions. Journal of Chemical Physics, 125(11).
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  Abstract: The mass-selected [(CO 2) 2(H 2O) m] - cluster anions are studied using a combination of photoelectron imaging and photofragment mass spectroscopy at 355 nm. Photoelectron imaging studies are carried out on the mass-selected parent cluster anions in the m=2-6 size range; photofragmentation results are presented for m=3-11. While the photoelectron images suggest possible coexistence of the CO 2-(H 2O) mCO 2 and (O 2CCO 2) -(H 2O) m parent cluster structures, particularly for m=2 and 3, only the CO 2- based clusters are both required and sufficient to explain all fragmentation pathways for m≥3. Three types of anionic photofragments are observed: CO 2-(H 2O) k, O -(H 2O) k, and CO 3-(H 2O) k, k≤m, with their yields varying depending on the parent cluster size. Of these, only CO 2-(H 2O) k can potentially result from (O 2CCO 2) -(H 2O) m parent structures, although an alternative mechanism, involving the dissociation and recombination of the CO 2- cluster core, is possible as well. The O -(H 2O) k and CO 3- (H 2O) k channels are believed to be triggered by the dissociation of the CO 2- cluster core. In the CO 3-(H 2O) k channel, seen only in the range of m=3-6, the CO 2- core dissociation is followed by an intracluster association of nascent O - with the solvent CO 2. This channel's absence in larger clusters (m > 6) is attributed to hindrance from the H 2O molecules. © 2006 American Institute of Physics.
• Mabbs, R., Pichugin, K., & Sanov, A. (2005). Dynamic molecular interferometer: Probe of inversion symmetry in I ₂^- photodissociation. Journal of Chemical Physics, 123(5).
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  Abstract: Time-resolved photoelectron imaging of negative ions is employed to examine 780-nm dissociation dynamics of I2-, emphasizing the effects of interference in time-resolved photoelectron angular distributions obtained with 390-nm probe. No energetic changes are observed after about 700 fs, but the evolution of the photoelectron anisotropy persists for up to 2.5 ps, indicating that the electronic wave function of the dissociating anion continues to evolve long after the asymptotic energetic limit of the reaction has been effectively reached. The time scale of the anisotropy variation corresponds to a fragment separation of the same order of magnitude as the de Broglie wavelength of the emitted electrons (λ=35 Å). These findings are interpreted by considering the effect of I2- inversion symmetry and viewing the dissociating anion as a dynamic molecular-scale "interferometer," with the electron waves emitted from two separating centers. The predictions of the model are in agreement with the present experiment and shed new light on previously published results [A. V. Davis, R. Wester, A. E. Bragg, and D. M. Neumark, J. Chem. Phys. 118, 999 (2003)]. © 2005 American Institute of Physics.
• Mabbs, R., Pichugin, K., & Sanov, A. (2005). Time-resolved imaging of the reaction coordinate. Journal of Chemical Physics, 122(17).
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  Abstract: Time-resolved photoelectron imaging of negative ions is employed to study the dynamics along the reaction coordinate in the photodissociation of IBr-. The results are discussed in a side-by-side comparison with the dissociation of I2-, examined under similar experimental conditions. The I2- anion, extensively studied in the past, is used as a reference system for interpreting the IBr- results. The data provide rigorous dynamical tests of the anion electronic potentials. The evolution of the energetics revealed in the time-resolved (780 nm pump, 390 nm probe) I2- and IBr- photoelectron images is compared to the predictions of classical trajectory calculations, with the time-resolved photoelectron spectra modeled assuming a variety of neutral states accessed in the photodetachment. In light of good overall agreement of the experimental data with the theoretical predictions, the results are used to construct an experimental image of the IBr- dissociation potential as a function of the reaction coordinate. © 2005 American Institute of Physics.
• Mabbs, R., Surber, E., & Sanov, A. (2005). Photoelectron anisotropy and channel branching ratios in the detachment of solvated iodide cluster anions. Journal of Chemical Physics, 122(5).
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  Abstract: Photoelectron spectra and angular distributions in 267 nm detachment of the I -ṡAr, I -ṡH 2O, I -ṡCH 3I, and I -ṡCH 3CN cluster anions are examined in comparison with bare I - using velocity-map photoelectron imaging. In all cases, features are observed that correlate to two channels producing either I( 2P 3/2) or I( 2P 1/2). In the photodetachment of I - and I -ṡAr, the branching ratios of the 2P 1/2 and 2P 3/2 channels are observed to be ≈0.4, in both cases falling short of the statistical ratio of 0.5. For I -ṡH 2O and I -ṡCH 3I, the 2P 1/2 to 2P 3/2 branching ratios are greater by a factor of 1.6 compared to the bare iodide case. The relative enhancement of the 2P 1/2 channel is attributed to dipole effects on the final-state continuum wave function in the presence of polar solvents. For I -ṡCH 3CN the 2P 1/2 to 2P 3/2 ratio falls again, most likely due to the proximity of the detachment threshold in the excited spin-orbit channel. The photoelectron angular distributions in the photodetachment of I -, I -ṡAr, I -ṡH 2O, and I -ṡCH 3CN are understood within the framework of direct detachment from I -. Hence, the corresponding anisotropy parameters are modeled using variants of the Cooper-Zare central-potential model for atomic-anion photodetachment. In contrast, I -ṡCH 3I yields nearly isotropic photoelectron angular distributions in both detachment channels. The implications of this anomalous behavior are discussed with reference to alternative mechanisms, affording the solvent molecule an active role in the electron ejection process. © 2005 American Institute of Physics.
• Sanov, A., Mabbs, R., Pichugin, K., & Sanov, A. M. (2005). Dynamic molecular interferometer: probe of inversion symmetry in I2(-) photodissociation. The Journal of chemical physics, 123(5).
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  Time-resolved photoelectron imaging of negative ions is employed to examine 780-nm dissociation dynamics of I2(-), emphasizing the effects of interference in time-resolved photoelectron angular distributions obtained with 390-nm probe. No energetic changes are observed after about 700 fs, but the evolution of the photoelectron anisotropy persists for up to 2.5 ps, indicating that the electronic wave function of the dissociating anion continues to evolve long after the asymptotic energetic limit of the reaction has been effectively reached. The time scale of the anisotropy variation corresponds to a fragment separation of the same order of magnitude as the de Broglie wavelength of the emitted electrons (lambda=35 A). These findings are interpreted by considering the effect of I2(-) inversion symmetry and viewing the dissociating anion as a dynamic molecular-scale "interferometer," with the electron waves emitted from two separating centers. The predictions of the model are in agreement with the present experiment and shed new light on previously published results [A. V. Davis, R. Wester, A. E. Bragg, and D. M. Neumark, J. Chem. Phys. 118, 999 (2003)].
• Sanov, A., Mabbs, R., Pichugin, K., & Sanov, A. M. (2005). Time-resolved imaging of the reaction coordinate. The Journal of chemical physics, 122(17).
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  Time-resolved photoelectron imaging of negative ions is employed to study the dynamics along the reaction coordinate in the photodissociation of IBr(-). The results are discussed in a side-by-side comparison with the dissociation of I(2) (-), examined under similar experimental conditions. The I(2) (-) anion, extensively studied in the past, is used as a reference system for interpreting the IBr(-) results. The data provide rigorous dynamical tests of the anion electronic potentials. The evolution of the energetics revealed in the time-resolved (780 nm pump, 390 nm probe) I(2) (-) and IBr(-) photoelectron images is compared to the predictions of classical trajectory calculations, with the time-resolved photoelectron spectra modeled assuming a variety of neutral states accessed in the photodetachment. In light of good overall agreement of the experimental data with the theoretical predictions, the results are used to construct an experimental image of the IBr(-) dissociation potential as a function of the reaction coordinate.
• Sanov, A., Mabbs, R., Surber, E., & Sanov, A. M. (2005). Photoelectron anisotropy and channel branching ratios in the detachment of solvated iodide cluster anions. The Journal of chemical physics, 122(5).
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  Photoelectron spectra and angular distributions in 267 nm detachment of the I(-)Ar, I(-)H(2)O, I(-)CH(3)I, and I(-)CH(3)CN cluster anions are examined in comparison with bare I(-) using velocity-map photoelectron imaging. In all cases, features are observed that correlate to two channels producing either I((2)P(3/2)) or I((2)P(1/2)). In the photodetachment of I(-) and I(-)Ar, the branching ratios of the (2)P(1/2) and (2)P(3/2) channels are observed to be approximately 0.4, in both cases falling short of the statistical ratio of 0.5. For I(-)H(2)O and I(-)CH(3)I, the (2)P(1/2) to (2)P(3/2) branching ratios are greater by a factor of 1.6 compared to the bare iodide case. The relative enhancement of the (2)P(1/2) channel is attributed to dipole effects on the final-state continuum wave function in the presence of polar solvents. For I(-)CH(3)CN the (2)P(1/2) to (2)P(3/2) ratio falls again, most likely due to the proximity of the detachment threshold in the excited spin-orbit channel. The photoelectron angular distributions in the photodetachment of I(-), I(-)Ar, I(-)H(2)O, and I(-)CH(3)CN are understood within the framework of direct detachment from I(-). Hence, the corresponding anisotropy parameters are modeled using variants of the Cooper-Zare central-potential model for atomic-anion photodetachment. In contrast, I(-)CH(3)I yields nearly isotropic photoelectron angular distributions in both detachment channels. The implications of this anomalous behavior are discussed with reference to alternative mechanisms, affording the solvent molecule an active role in the electron ejection process.
• Sanov, A., Surber, E., Mabbs, R., Habteyes, T., & Sanov, A. M. (2005). Photoelectron imaging of hydrated carbon dioxide cluster anions. The journal of physical chemistry. A, 109(20).
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  The effects of homogeneous and heterogeneous solvation on the electronic structure and photodetachment dynamics of hydrated carbon dioxide cluster anions are investigated using negative-ion photoelectron imaging spectroscopy. The experiments are conducted on mass-selected [(CO(2))(n)()(H(2)O)(m)()](-) cluster anions with n and m ranging up to 12 and 6, respectively, for selected clusters. Homogeneous solvation in (CO(2))(n)()(-) has minimal effect on the photoelectron angular distributions, despite dimer-to-monomer anion core switching. Heterogeneous hydration, on the other hand, is found to have the marked effect of decreasing the photodetachment anisotropy. For example, in the [CO(2)(H(2)O)(m)()](-) cluster anion series, the photoelectron anisotropy parameter falls to essentially zero with as few as 5-6 water molecules. The analysis of the data, supported by theoretical modeling, reveals that in the ground electronic state of the hydrated clusters the excess electron is localized on CO(2), corresponding to a (CO(2))(n)()(-).(H(2)O)(m)() configuration for all cluster anions studied. The diminishing anisotropy in the photoelectron images of hydrated cluster anions is proposed to be attributable to photoinduced charge transfer to solvent, creating transient (CO(2))(n)().(H(2)O)(m)()(-) states that subsequently decay via autodetachment.
• Surber, E., Mabbs, R., Habteyes, T., & Sanov, A. (2005). Photoelectron imaging of hydrated carbon dioxide cluster anions. Journal of Physical Chemistry A, 109(20), 4452-4458.
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  PMID: 16833780;Abstract: The effects of homogeneous and heterogeneous solvation on the electronic structure and photodetachment dynamics of hydrated carbon dioxide cluster anions are investigated using negative-ion photoelectron imaging spectroscopy. The experiments are conducted on mass-selected [(CO 2) n(H 2O) m] - cluster anions with n and m ranging up to 12 and 6, respectively, for selected clusters. Homogeneous solvation in (CO 2) n- has minimal effect on the photoelectron angular distributions, despite dimer-to-monomer anion core switching. Heterogeneous hydration, on the other hand, is found to have the marked effect of decreasing the photodetachment anisotropy. For example, in the [CO 2(H 2O) m] - cluster anion series, the photoelectron anisotropy parameter falls to essentially zero with as few as 5-6 water molecules. The analysis of the data, supported by theoretical modeling, reveals that in the ground electronic state of the hydrated clusters the excess electron is localized on CO 2, corresponding to a (CO 2) n-·(H 2O) m configuration for all cluster anions studied. The diminishing anisotropy in the photoelectron images of hydrated cluster anions is proposed to be attributable to photoinduced charge transfer to solvent, creating transient (CO 2) n·(H 2O) m- states that subsequently decay via autodetachment. © 2005 American Chemical Society.
• Mabbs, R., Pichugin, K., Surber, E., & Sanov, A. (2004). Time-resolved electron detachment imaging of the I ^- channel in I ₂Br photodissociation. Journal of Chemical Physics, 121(1), 265-272.
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  PMID: 15260544;Abstract: The use of time-resolved negative-ion photoelectron imaging spectroscopy for the evolution of the I - channel of I 2Br - photodissociation was analyzed. The 388 nm pump-388 nm probe photoelectron images showed details of the electronic structure transformation from the excited molecular anion to the atomic-anion fragment. It was stated that the dynamics of the photodissociation can be divided into two stages. In the first stage, the excess-electron wave function transforms from a molecular orbital to a localized atomic orbital. The second stage involves exit-channel dynamics where fragment interactions persists.
• Mabbs, R., Surber, E., Velarde, L., & Sanov, A. (2004). Effects of solvation and core switching on the photoelectron angular distributions from (CO ₂) _n^- and (CO ₂) _n^-·H ₂O. Journal of Chemical Physics, 120(11), 5148-5154.
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  PMID: 15267384;Abstract: The photodetachment of carbon-dioxide cluster anions with a special emphasis on the photoelectron angular distributions (PAD) was investigated. The imaging approach allowed a systematic examination of PAD which reflected the orbital nature of the different core structures. The study of PAD showed that solvation by several carbon-dioxide molecular and/or one water molecule has only moderate effect on the excess-electron orbitals. The observed PAD character was reconciled with the symmetry properties of the parent molecular orbitals, and the results showed remarkable similarities between the monomer and dimer anion cluster-core types.
• Sanov, A., & Lineberger, W. C. (2004). Cluster anions: Structure, interactions, and dynamics in the sub-nanoscale regime. Physical Chemistry Chemical Physics, 6(9), 2018-2032.
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  Abstract: This article reflects on several recent advances in the studies of structure and dynamics of cluster anions, both as unique species and as a conceptual bridge between isolated molecules and the condensed phase. Applications involving photofragment and photodetachment spectroscopy, as well as femtosecond time-resolved experiments, are described. Special emphasis is given to the effects of microscopic solvation on the electronic structure and reactivity of negative ions in heterogeneous and homogeneous cluster environments. Several recent breakthroughs in experimental methodology are outlined, in particular the application of photoelectron imaging to the studies of molecular cluster anions.
• Sanov, A., Mabbs, R., Pichugin, K., Surber, E., & Sanov, A. M. (2004). Time-resolved electron detachment imaging of the I- channel in I2Br- photodissociation. The Journal of chemical physics, 121(1).
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  The evolution of the I(-) channel in I(2)Br(-) photodissociation is examined using time-resolved negative-ion photoelectron imaging spectroscopy. The 388 nm photodetachment images obtained at variable delays following 388 nm excitation reveal the transformation of the excess electron from that belonging to an excited trihalide anion to that occupying an atomic orbital localized on the I(-) fragment. With increasing pump-probe delay, the corresponding photoelectron band narrows on a approximately 300 fs time scale. This trend is attributed to the localization of the excess-electron wave function on the atomic-anion fragment and the establishment of the fragment's electronic identity. The corresponding band position drifts towards larger electron kinetic energies on a significantly longer, approximately 1 ps, time scale. The gradual spectral shift is attributed to exit-channel interactions affecting the photodetachment energetics, as well as the photoelectron anisotropy. The time-resolved angular distributions are analyzed and found consistent with the formation of the asymptotic I(-) fragment.
• Sanov, A., Mabbs, R., Surber, E., Velarde, L., & Sanov, A. M. (2004). Effects of solvation and core switching on the photoelectron angular distributions from (CO2)n(-) and (CO2)(n)(-).H2O. The Journal of chemical physics, 120(11).
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  Photoelectron images are recorded in the photodetachment of two series of cluster anions, (CO(2))(n)(-), n=4-9 and (CO(2))(n)(-).H(2)O, n=2-7, with linearly polarized 400 nm light. The energetics of the observed photodetachment bands compare well with previous studies, showing evidence for switching between two anionic core structures: The CO(2)(-) monomer and covalent (CO(2))(2)(-) dimer anions. The systematic study of photoelectron angular distributions (PADs) sheds light on the electronic structure of the different core anions and indicates that solvation by several CO(2) molecules and/or one water molecule has only moderate effect on the excess-electron orbitals. The observed PAD character is reconciled with the symmetry properties of the parent molecular orbitals. The most intriguing result concerns the PADs showing remarkable similarities between the monomer and dimer anion cluster-core types. This observation is explained by treating the highest-occupied molecular orbital of the covalent dimer anion as a linear combination of two spatially separated monomeric orbitals.
• Mabbs, R., Surber, E., & Sanov, A. (2003). An experimental manifestation of distinct electronic-structural properties of covalent dimer anions of CO₂ and CS₂. Chemical Physics Letters, 381(3-4), 479-485.
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  Abstract: Photoelectron imaging is employed to examine the monomer and dimer anions of CO2 and CS2, either isolated or embedded in clusters. In both cases, photodetachment from the dimer anion is compared to the corresponding monomer. The comparison of CS2- and (CS 2)2- images reveals dissimilar bands attributed to detachment from different orbitals/species. Yet the photoelectron angular distributions obtained from the monomer and dimer anion cores of (CO2)n- clusters are surprisingly similar. These findings are reconciled with the qualitatively different electronic-structural properties of (CS2)2- and (CO2)2-. © 2003 Elsevier B.V. All rights reserved.
• Mabbs, R., Surber, E., & Sanov, A. (2003). Photoelectron imaging of negative ions: Atomic anions to molecular clusters. Analyst, 128(6), 765-772.
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  PMID: 12866901;Abstract: The negative ion photoelectron imaging technique is illustrated using two relatively simple atomic and molecular anion systems, and then applied to the study of a cluster system. Photoelectron images of I- and CS2- at 267 nm and 800 nm respectively are presented. Photoelectron spectra and angular distributions are obtained from the images and the concepts underlying these and their interpretation are outlined. The imaging technique is then applied to (CS2)n- (n = 2-4) cluster anions, for which 400 nm images are presented. Features of these images are highlighted and discussed with reference to solvation effects and structural properties of the cluster anionic moiety. Photoelectron signatures of different forms of the cluster core are discussed. These core structures are anionic monomer units solvated by the remaining n - 1 CS2 molecules or covalent dimer units solvated by the remaining n - 2 molecules. Images of the n = 2 anion at 400, 530 and 800 nm reveal information about the electron detachment processes within the different cluster types and both direct detachment and autodetachment are seen. The direct transitions are seen from clusters with either core type, while autodetachment is only seen from clusters with the covalent dimer core. The imaging work also reveals evidence of a previously unreported electronic transition within the direct detachment band due to the covalently bound core type.
• Sanov, A., Mabbs, R., Surber, E., & Sanov, A. M. (2003). Photoelectron imaging of negative ions: atomic anions to molecular clusters. The Analyst, 128(6).
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  The negative ion photoelectron imaging technique is illustrated using two relatively simple atomic and molecular anion systems, and then applied to the study of a cluster system. Photoelectron images of I- and CS2- at 267 nm and 800 nm respectively are presented. Photoelectron spectra and angular distributions are obtained from the images and the concepts underlying these and their interpretation are outlined. The imaging technique is then applied to (CS2)n - (n = 2-4) cluster anions, for which 400 nm images are presented. Features of these images are highlighted and discussed with reference to solvation effects and structural properties of the cluster anionic moiety. Photoelectron signatures of different forms of the cluster core are discussed. These core structures are anionic monomer units solvated by the remaining n - 1 CS2 molecules or covalent dimer units solvated by the remaining n - 2 molecules. Images of the n = 2 anion at 400, 530 and 800 nm reveal information about the electron detachment processes within the different cluster types and both direct detachment and autodetachment are seen. The direct transitions are seen from clusters with either core type, while autodetachment is only seen from clusters with the covalent dimer core. The imaging work also reveals evidence of a previously unreported electronic transition within the direct detachment band due to the covalently bound core type.
• Sanov, A., Surber, E., & Sanov, A. M. (2003). Imaging of direct photodetachment and autodetachment of (OCS)2-: excited-state dynamics of the covalent dimer anion. Physical review letters, 90(9).
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  We report a photoelectron imaging study of (OCS)-2 and compare the results to OCS-.H2O. Two electron-emission mechanisms are observed for the dimer anion: direct photodetachment and autodetachment, while OCS-.H2O exhibits only the direct mechanism. The results provide evidence of covalent (OCS)-2 coexisting with the OCS-.OCS cluster anion. The autodetachment originating from the covalent species is modeled as thermionic emission transpiring in the regime of fragmentation. The bulk statistical model is found applicable to the small anion due to the availability of low-lying excited states.
• Surber, E., & Sanov, A. (2003). Imaging of direct photodetachment and autodetachment of (OCS)₂^-: Excited-state dynamics of the covalent dimer anion. Physical Review Letters, 90(9), 093001/1-093001/4.
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  Abstract: Ion clusters are excellent vehicles for the study of the transition between the gas-phase and bulk properties of matter as they allow examination of molecular ensembles under well-defined conditions. Thus, photoelectron imaging experiments in which both the molecular and "bulk" properties are manifest in the excitation of a small disintegrating anion is reported. The results show that th applicability of the bulk description to such a highly dynamic system is dependent on the electronic structure, particularly the availability of low-lying excited states.
• Surber, E., & Sanov, A. (2003). Photoelectron imaging of carbonyl sulfide cluster anions: Isomer coexistence and competition of excited-state decay mechanisms. Journal of Chemical Physics, 118(20), 9192-9200.
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  Abstract: An attempt was made to decipher the structure of (OCS)n- cluster anions using photoelectron spectroscopy. The employed imaging approach allowed the simultaneous examination of energy spectra and angular distributions of both fast and slow photoelectrons. The results provide evidence of the coexistence of electronic and structural isomers of (OCS)n- with the OCS- and covalently bound (OCS)2- cluster cores.
• Surber, E., Mabbs, R., & Sanov, A. (2003). Probing the electronic structure of small molecular anions by photoelectron imaging. Journal of Physical Chemistry A, 107(40), 8215-8224.
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  Abstract: We outline the methodology of negative-ion photoelectron imaging and general aspects of interpretation of the results using the CS2- and S2-1 anions as model systems. The CS2- images are recorded using 800, 530, 400, and 267 nm photons. The observed transitions result in the formation of CS2 in the X 1Σ1+, a 3B2, b 3A2, and A 1A2 states. The S2- measurements are carried out at the same wavelengths with the exception of 800 nm. The resulting images reveal the detachment transitions assigned to the X 3Σg-, a 1Δg-, b 1Σg+, c 1Σu-, and A′ 3Δu states of the neutral. The choice of detachment wavelengths serves as a "zoom" selectively focusing on chosen transitions, in some cases allowing the observation of their vibrational structure. The photoelectron spectra and angular distributions obtained from the images are used to discuss the electronic structure and detachment dynamics. In particular, two approaches to interpreting the angular distributions are discussed. One method employs the Cooper-Zare central-potential model adapted to the molecular case. It considers an expansion of the parent orbital in the basis of single-center atomic-orbital functions, for which the partial waves comprising the ejected are determined. The application of this model to molecular anions is straightforward, if the parent molecular orbital resembles an atomic orbital, which is the case for S2-, but not CS2-. In the latter case, a different qualitative approach is proposed, which (1) relies upon the electric-dipole approximation and group theory for the determination of the detached electron wave function symmetry, (ii) restricts the analysis to symmetry (electric dipole) allowed s and p partial waves, and (iii) qualitatively treats the orientation averaging by considering only a few "principal" molecular orientations. The results provide a foundation for the qualitative interpretation of anion photoelectron images.
• Sanov, A., & Lineberger, W. C. (2002). Dynamics of cluster anions: A detailed look at condensed-phase interactions. PhysChemComm, 5, 165-177.
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  Abstract: This Perspective reflects on several recent advances in the studies of structure and dynamics of cluster anions, bridging the gap between 'cluster' and condensed phases. Applications involving photofragment and photodetachment spectroscopy, as well as femtosecond time-resolved experiments, are described. Special emphasis is given to the effects of microscopic solvation on the electronic structure and reactivity of negative ions in heterogeneous and homogeneous cluster environments. Some recent breakthroughs in experimental methodology are also outlined, in particular the application of photofragment and photoelectron methods and the imaging technique to the studies of molecular cluster anions. © The Royal Society of Chemistry 2002.
• Surber, E., & Sanov, A. (2002). Photoelectron imaging spectroscopy of molecular and cluster anions: CS₂^- and OCS^- (H₂O)_1,2. Journal of Chemical Physics, 116(14), 5921-5924.
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  Abstract: Photoelectron imaging was used to study molecular and cluster anions on the examples of CS2- and OCS-(H2O)1,2. Results for OCS-(H2O)1,2 were compared to that of CS2-. The adiabatic electron affinity of OCS was estimated at -0.04 eV, while the VDEs of OCS-·H2 and OCS- were determined to be 2.07±0.07 and 2.53±0.07 eV, respectively.
• Surber, E., Ananthavel, S. P., & Sanov, A. (2002). Nonexistent electron affinity of OCS and the stabilization of carbonyl sulfide anions by gas phase hydration. Journal of Chemical Physics, 116(5), 1920-1929.
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  Abstract: The formation of the [(OCS)n(H2O)k]-- (n≥1,n+k≥2) cluster ions was followed. OCS-·H2O was found to be the most interesting species in view of the near absence of the seemingly simpler OCS- in the same ion source. The presence of the monohydrated anions was attribued to the stabilization of OCS- by hydration and served as unambiguous proof that the intra-cluster formation of OCS- monomers is part of the [(OCS)n(H2O)k]- dynamics.
• Conroy, D., Aristov, V., Feng, L., Sanov, A., & Reisler, H. (2001). Competitive pathways via nonadiabatic transitions in photodissociation. Accounts of Chemical Research, 34(8), 625-632.
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  PMID: 11513569;Abstract: Photodissociation processes of molecules and radicals involving multiple pathways and nonadiabatic crossings are studied using the photofragment imaging technique and the core-sampling version of time-of-flight spectroscopy. Capabilities and challenges are illustrated by two systems. The isocyanic acid system demonstrates how interactions among potential energy surfaces can change during dissociation. The hydroxymethyl photodecomposition system highlights Rydberg-valence interactions common in free radicals. The cross-fertilization between theory and experiment is emphasized.
• Sanov, A., Faeder, J., Parson, R., & Lineberger, W. C. (1999). Spin-orbit coupling in I·CO₂ and I·OCS van der Waals complexes: Beyond the pseudo-diatomic approximation. Chemical Physics Letters, 313(5-6), 812-819.
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  Abstract: We investigate theoretically the electronic structure of I·CO2 and I·OCS van der Waals complexes including spin-orbit interaction. For the T-shaped geometry of I·CO2, we calculate the potentials using a fully polyatomic treatment and compare the results to the widely used pseudo-diatomic approximation. The latter becomes increasingly invalid at I-CO2 distances shorter than 4 Å. We calculate the potentials of linear I·SCO and I·OCS, and analyze the zero-order electronic structure of nonlinear I·OCS. We also discuss the validity of treating the spin-orbit interaction in I·CO2 and I·OCS as an atomic property of iodine and find this approximation justified at characteristic van der Waals distances.
• Sanov, A., Sanford, T., Butler, L. J., Vala, J., Kosloff, R., & Lineberger, W. C. (1999). Photodissociation dynamics of gas-phase BrICl^- and IBr₂^- anions. Journal of Physical Chemistry A, 103(49), 10244-10254.
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  Abstract: This work is the first study of mixed trihalide anions in the gas phase. We report the photochemistry and photodissociation dynamics of BrICl- and IBr2- anions near 400 nm. For BrICl-, two major photodissociation channels yielding IBr- and ICl- products are observed. The photodissociation of IBr2- yields primarily IBr- anion products. No BrCl- and Br2- fragments are formed from BrICl- and IBr2-, respectively, and the parent anions are ascribed linear [Br-I-Cl]- and [Br-I-Br]- structures, in agreement with ab initio calculations. The IBr- and ICl- products are produced in highly vibrationally excited states. We observe fragment vibrational coherence in femtosecond pump-probe measurements by probing nascent IBr- via its photodissociation to I- + Br. The IBr- vibrational periods observed in the photodissociation of BrICl- correspond to excitations of ∼80% of the IBr- dissociation energy (Do). In IBr2- dissociation, the average vibrational excitation of IBr- is 84-91% of Do. The high degrees of vibrational excitation raise questions about the effect of the proximity to the dissociation threshold on the dynamics of coherent wave packets. We analyze these dynamics analytically and by means of wave packet simulations and observe smearing of expected wave packet revivals. © 1999 American Chemical Society.
• Sanov, A., Sanford, T., Nandi, S., & Lineberger, W. C. (1999). Spin-orbit relaxation and recombination dynamics in I^-₂(CO₂)_n and I^-₂(OCS)_n cluster ions: A new type of photofragment caging reaction. Journal of Chemical Physics, 111(2), 664-675.
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  Abstract: We report a new type of photofragment caging reaction that is only possible because of the strong solvent-induced perturbation of the inherent electronic structure of the chromophore. The photoexcitation of I-2 at 395 nm promotes it to a dissociative state correlating with I-+I*(2P1/2), the only near-ultraviolet dissociation channel for unsolvated I-2. In I-2 (CO2)n and I-2(OCS)n clusters, interaction with the solvent is observed to result in extremely fast spin-orbit relaxation. In general, we detect three reaction pathways: (1) direct dissociation of the chromophore to I-+I*(2P1/2); (2) the I-2→I-+I* dissociation, followed by spin-orbit quenching leading to I-+I(2P3/2) products; and (3) the I-2→I-+I* dissociation, followed by spin-orbit quenching and I-+I(2P3/2)→I-2 recombination and vibrational relaxation. We present experimental evidence of the spin-orbit relaxation and caging and discuss possible mechanisms. The results include: the measured translational energy release in 395 nm photodissociation of unsolvated I-2, indicating that solvation-free dissociation proceeds exclusively via the I-I* channel; ionic product distributions in the photodissociation of size-selected I-2(CO2)n and I-2(OCS)n clusters at the same wavelength, indicating the above three reaction channels; and ultrafast pump-probe measurements of absorption recovery, indicating picosecond time scales of the caging reaction. We rule out the mechanisms of spin-orbit quenching relying on I*-solvent interactions without explicitly considering the perturbed electronic structure of I-2. Instead, as described by Delaney et al. (companion paper), the spin-orbit relaxation occurs by electron transfer from I- to I*(2P1/2), giving I(2P3/2)+I-. The 0.93 eV gap between the initial and final states in this transition is bridged by differential solvation due to solvent asymmetry. Favorable comparison of our experimental results and the theoretical simulations of Delaney et al. yield confidence in the mechanism and provide understanding of the role of cluster structure in spin-orbit relaxation and recombination dynamics. © 1999 American Institute of Physics.
• Zyrianov, M., Sanov, A., Droz-Georget, T., & Reisler, H. (1999). Photoinitiated decomposition of HNCO near the H+NCO threshold: Centrifugal barriers and channel competition. Journal of Chemical Physics, 110(22), 10774-10783.
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  Abstract: The decomposition of jet-cooled HNCO is investigated near the H+NCO channel threshold [D0(H+NCO)=38 370 cm-1]. Dissociation to H+NCO at energies 17-411 cm-1 above D0(H+NCO) proceeds on the ground potential energy surface (S0), apparently without a barrier. The rotational state distributions of the NCO(X2Π3/2,0010) fragment are well described by phase space theory (PST), provided that dynamical constraints are included. These constraints are associated with long range (4-7 Å) centrifugal barriers, which are significant even near threshold because of the small reduced mass of H+NCO, and result in a fraction of energy deposited in fragment rotation much smaller than predicted by unconstrained PST. The influence of orientation averaging on the attractive, long-range part of the potential is discussed, and it is argued that angular averaging with respect to the center of mass of the rotating polyatomic fragment results in a shift in the effective potential origin, accompanied by an attenuation of the magnitude of the potential compared to its value for fixed H-N distance. Following initial S1(1A″)←S0(1A′) excitation and internal conversion to S0, HNCO(S0) decays both via unimolecular decomposition of H+NCO and intersystem crossing to the dissociative first triplet state, T1 [yielding NH(X3∑-)+ CO products]. The competition between the two processes is interrogated by monitoring changes in the relative yields of NCO and NH(X3∑-) as a function of excitation energy. It is concluded that near D0(H+NCO), the S0→T1 intersystem crossing rate is several-fold faster than the H+NCO unimolecular decomposition rate. © 1999 American Institute of Physics.
• Nandi, S., Sanov, A., Delaney, N., Faeder, J., Parson, R., & Lineberger, W. C. (1998). Photodissociation of I₂^-(OCS)_n cluster ions: Structural implications. Journal of Physical Chemistry A, 102(45), 8827-8835.
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  Abstract: We report product distributions from the photodissociation of I2-(OCS)n (n = 1-26) cluster ions at 790 and 395 nm and discuss implications concerning the structure of these clusters. The experimental results are paralleled by a theoretical investigation of I2-(OCS)n structures. The 790 and 395 nm transitions in I2- access dissociative excited states that correlate with the I- + I(2P3/2) and I- + I*(2P1/2) products, respectively. Photoabsorption by I2-(OCS)n clusters at 790 nm results in "uncaged" I-(OCS)k and "caged" I2-(OCS)k fragments. The 395 nm excitation leads, in general, to three distinct pathways: (1) I2- dissociation on the I- + I*(2P1/2) spin-orbit excited asymptote, competing with the solvent-induced spin-orbit relaxation of I*(2P1/2) followed by either (2) I2- dissociation on the I- + I(2P3/2asymptote or (3) I2- recombination. Pathways 1 and 2 result in a bimodal distribution of the uncaged I-(OCS)k fragments that energetically correlate with the two spin-orbit states of the escaping I atom. The I + I- caging efficiency is determined as a function of the number of solvent OCS molecules at both excitation wavelengths studied. At 790 nm, 100% caging of I2- is achieved for n ≥ 17. For 395 nm excitation, addition of the 17th OCS molecule to I2-(OCS)16 results in a steplike increase in the caging efficiency from 0.25 to 0.68. These results suggest that the first solvent shell around I2- is comprised of 17 OCS molecules. Results of theoretical calculations of the lowest-energy I2-(OCS)n cluster structures support this conclusion. The roles of different dominant electrostatic moments of OCS and CO2 in defining the I2-(OCS)n and I2-(CO2)n cluster structures are discussed, based on comparison of the photofragment distributions.
• Sanov, A., & Lineberger, W. C. (1998). Incoherent control of photodissociation pathways in I₂^- cluster ions. Proceedings of SPIE - The International Society for Optical Engineering, 3271, 188-195.
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  Abstract: Detailed time-resolved photodissociation and caging dynamics are reported for an I2-(OCS)11 model system. The observed product channel-dependent nuclear coherence in the dissociated chromophore reflects complex dynamics of the solvent cage. The evolving pump-probe product distribution offers the possibility of incoherent control of two-photon dissociation pathways by appropriately delaying the probe laser pulse. As an example of such control, I2-(OCS)2 is produced most effectively by a limited set of pump-probe excitations. We emphasize generality of these results that relate to caging dynamics in any cluster ions.
• Sanov, A., Lineberger, W. C., & Jordan, K. D. (1998). Electronic structure of (CS₂)₂^-. Journal of Physical Chemistry A, 102(15), 2509-2511.
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  Abstract: We investigate theoretically the electronic structure of (CS2)2-. Five different low-lying forms of the anion are identified. These include the ion-molecule complex and a covalently bonded dimer anion, both of which have electronic configurations consistent with electron attachment to the LUMO of the van der Waals dimer. The other species, including the most stable form of the anion, all have cyclic structures and can be viewed as arising from electron capture by cyclic covalently bonded neutral dimers, with electronic configurations doubly excited with respect to the van der Waals dimer. Vertical detachment energies are calculated for the various anions and lead us to reassign the features in the observed photoelectron spectra of (CS2)2-.
• Sanov, A., Nandi, S., & Lineberger, W. C. (1998). Transient solvent dynamics and incoherent control of photodissociation pathways in I^-₂ cluster ions. Journal of Chemical Physics, 108(13), 5155-5158.
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  Abstract: Detailed time-resolved photodissociation and caging dynamics in clusters are studied using I-2(OCS)11 as a model system. We report new observations of product channel-dependent properties of nuclear coherence in the dissociated chromophore, reflecting complex dynamics of the solvent cage. The coherence feature is most pronounced in the caged two-photon channels and its relative amplitude increases with the product size. Shorter delays, on the time scale of coherent I⋯I- motion, favor larger products, allowing for incoherent control of two-photon dissociation pathways by appropriately timing the two laser pulses. As an example of such control, I-2(OCS)2 is produced most effectively by a limited set of pump-probe excitations at short delays. We emphasize generality of these results that relate to caging dynamics in any cluster ions. © 1998 American Institute of Physics.
• Sanov, A., Nandi, S., Jordan, K. D., & Lineberger, W. C. (1998). Photochemistry of (OCS)_n^- cluster ions. Journal of Chemical Physics, 109(4), 1264-1270.
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  Abstract: We report the photochemistry of (OCS)n- cluster ions following 395 nm (n=2-28) and 790 nm (n=2-4) excitation. In marked contrast to (CO2)n-, extensive bond breaking and rearrangement is observed. Three types of ionic products are identified: S2-(OCS)k, S-(OCS)k/OCS2-(OCS)k-1, and (OCS)k-. For n
• Droz-Georget, T., Zyrianov, M., Sanov, A., & Reisler, H. (1997). Photodissociation of HNCO: Three competing pathways. Berichte der Bunsengesellschaft/Physical Chemistry Chemical Physics, 101(3), 469-477.
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  Abstract: The unimolecular decomposition of expansion-cooled isocyanic acid (HNCO) via channels (1) 3NH + CO, (2) H+NCO, and (3) 1NH + CO [where 3NH and 1NH denote NH(X3∑-) and NH(a1δ), respectively] has been investigated following photoexcitation to the S1(1A″) state in two energy regimes: (i) in the region of the 1NH + CO threshold (41700-45500cm-1; 240-220nm), and (ii) ∼ 3200cm-1 above D0(1NH + CO), at around 46000 cm-1 (217.6nm). Several complementary experiments are presented: NCO, 3NH and 1NH photofragment yield spectra and relative 1NH/3NH branching ratios are obtained by laser induced fluorescence (LIF); photofragment ion imaging is used to record CO angular recoil distributions, and 1NH rotational distributions correlated with specific CO(v,J) levels. HNCO excited to S, undergoes complex dynamics reflecting simultaneous decomposition on several potential energy surfaces, and including internal conversion (1C) and intersystem crossing (ISC). In energy region (i), a progressive loss of structure in the 3NH yield spectrum is observed above the opening of channel (3), and is interpreted as the imprint of short-time dynamics characteristic of the ISC step. State selectivity in the photodissociation is revealed by comparing the photofragment yield spectra of the three channels. In region (ii), product state distributions for channel (3) exhibit clear dynamical signatures, as expected for dissociation on S1 At low excess energies channel (2) derives from dissociation on S0, but the respective roles of S0 and S1 at higher energies are not well established yet. The results are discussed in terms of vibronic levels of mixed electronic character coupled directly or via radiationless decay to the various continua. The competition between the different processes depends sensitively on photolysis energy and excitation conditions. © VCH Verlagsgesellschaft mbH. 1997.
• Sanov, A., Arnold, D. W., Korolik, M., Ferkel, H., Bieler, C. R., Capellos, C., Wittig, C., & Reisler, H. (1997). Collision-Induced Dissociation of Highly Excited NO₂ in the Gas Phase and on MgO (100) Surfaces. ACS Symposium Series, 678, 291-303.
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  Abstract: Collision-induced dissociation (CID) of NO2 in highly excited mixed 2A1/2B2 states is studied in crossed molecular beams at collision energies of ∼2000 cm-1 and on crystalline MgO(100) at collision energies of ∼2000 and 4400 cm-1. The yield spectra obtained by scanning the excitation laser wavelength while monitoring NO fragments show features identical to those in the fluorescence excitation spectrum of NO2, but the yield of CID decreases exponentially with the increase of the amount of energy required to reach the threshold for the monitored NO state. The results are discussed in terms of a mechanism in which highly excited NO2 undergoes further activation by collisions, followed by unimolecular decomposition. The NO product spin-orbit excitations are sensitive to the chemical identity of the collider and bear the imprints of exit-channel interactions, which are more significant on the MgO(100) surface than in the gas-phase.
• Sanov, A., Droz-Georget, T., Zyrianov, M., & Reisler, H. (1997). Photofragment imaging of HNCO decomposition: Angular anisotropy and correlated distributions. Journal of Chemical Physics, 106(17), 7013-7022.
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  Abstract: Photodissociation of jet-cooled isocyanic acid has been examined by photofragment ion imaging of H(D) from H(D)NCO and CO from HNCO, and by laser induced fluorescence (LIF) of NH(a 1Δ) from HNCO. Only modest recoil anisotropy is observed in the H+NCO channel at 243.1 nm (β=-0.13±0.05), while the D+NCO channel at approximately the same wavelength reveals no anisotropy (β=0.00±0.05), confirming that the dissociation of H(D)NCO from the opening of the H(D) channel proceeds via vibrational predissociation on the S0(1A′) surface. In contrast, substantial anisotropy (β=-0.66±0.08) is observed in the NH(a 1Δ)+CO channel at 230.1 nm, but this value can correspond to dissociation on either S0 or S1. The photolysis region between 243 and 230 nm thus appears important in providing clues to the dissociation mechanism and the competition between different potential energy surfaces. At 217.6 nm, product state distributions exhibit clear dynamical biases. CO is produced in both v=0 and v=1, while NH(a 1Δ) distributions correlated with different rovibrational levels of CO, although different in shape, are always cold, consistent with the global NH distribution measured by LIF. The NH distributions indicate dissociation on S1(1A″), and can be described by Franck-Condon mapping of transition state wave functions in the HNC bending coordinate without additional torque, implying little anisotropy in the potential along that coordinate. On the other hand, a larger torque is manifest in the CO rotational distribution. Although at 217.6 nm the dissociation is likely to be dominated by decomposition on S1, competition with radiationless decay is still manifest. From analysis .of the CO photofragment velocity distribution at 230.1 nm, the NH(a 1Δ)+CO dissociation threshold is determined at 42 765 ±25 cm-1. © 1997 American Institute of Physics.
• Bieler, C. R., Sanov, A., Capellos, C., & Reisler, H. (1996). Molecular beams studies of the dissociation of highly excited NO₂ induced by molecular colliders. Journal of Physical Chemistry, 100(10), 3882-3887.
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  Abstract: NO2 in high vibrational levels was prepared in a pulsed molecular beam by laser excitation of the mixed 12A1/22B2 state to energies hv below dissociation threshold D0, D0 - hv = 0-500 cm-1. The beam of excited molecules was crossed with pulsed, neat molecular beams of HCl, CO2, N2O, and NH3 at relative collision energies of ∼2000 cm-1, and the NO produced by collision-induced dissociation (CID) was detected state-selectively. The CID yield spectra obtained by monitoring specific NO rotational levels while scanning the excitation wavelength show spectral features identical with those in the fluorescence excitation spectrum of NO2. The yield of the CID products, however, decreases exponentially (compared with the fluorescence spectrum) with the increase of the amount of energy required to reach the threshold of appearance of the monitored NO state. The average energy transferred per activating collision with polyatomic colliders is in the range 130-200 cm-1, having values similar to or lower than those for diatomic and atomic colliders. This is in contrast to deactivating collisions, in which polyatomic colliders are in general more effective. The results are discussed in terms of a mechanism in which the NO2 molecules are activated by impulsive collisions creating a distribution of molecules in quantum states above D0 whose populations diminish exponentially with energy. The collisional activation is followed by unimolecular decomposition. The differences between the activation and deactivation pathways are rationalized in terms of the number of degrees of freedom available for energy transfer in each channel. © 1996 American Chemical Society.
• Mikhaylichenko, K., Riehn, C., Valachovic, L., Sanov, A., & Wittig, C. (1996). Unimolecular decomposition of NO₃: The NO+O₂ threshold regime. Journal of Chemical Physics, 105(16), 6807-6817.
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  Abstract: The unimolecular decomposition of expansion-cooled NO3 has been investigated in the threshold regime of the NO+O2 channel. Photoexcitation in the region 16 780-17 090 cm-1 (596-585 nm) prepares ensembles of molecular eigenstates, each of which is a mixture of the B 2E′ bright state and lower electronic states. The X 2A′2 ground state is believed to be the probable terminus of 2E′ radiationless decay, though participation of A 2E″ is also possible. For these photon energies, unimolecular decomposition occurs exclusively via the NO+O2 channel, and NO yield spectra and state distributions have been obtained. The yield spectra are independent of the rotational state monitored, as expected for a large reverse barrier. The state distributions are insensitive to the photolysis photon energy and can be rationalized in terms of dynamical bias. The NO yield goes to zero rapidly above the O+NO2 threshold (17 090±20 cm-1). Because of tunneling, the NO+O2 channel does not have a precise threshold; the value 16 780 cm-1 is the smallest photon energy that yielded signals under the present conditions. Very small decomposition rates were obtained via time-domain measurements in which reactive quenching of long-lived NO3 fluorescence was observed. The rates varied from 1×104 at 16780 cm-1 to 6×107 s-1 at 16 880 cm-1, and their collision free nature was confirmed experimentally. These data were fitted by using a one-dimensional tunneling model for motion along the reaction coordinate combined with the threshold Rice-Ramsperger-Kassel-Marcus (RRKM) rate. The top of the NO+O2 barrier is estimated to lie at 16 900 ± 15 cm-1. Translational energy measurements of specific NO (X 2∏Ω,v,J) levels showed that O2 is highly excited, with a population inversion extending to energies above the a 1Δg threshold, in agreement with previous work. It is possible that the main O2 product is X 3∑g-, though some participation of a 1Δg cannot be ruled out. Within the experimental uncertainty, b 1∑g+ is not produced. © 1996 American Institute of Physics.
• Zyrianov, M., Droz-Georget, T., Sanov, A., & Reisler, H. (1996). Competitive photodissociation channels in jet-cooled HNCO: Thermochemistry and near-threshold predissociation. Journal of Chemical Physics, 105(18), 8111-8116.
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  Abstract: The photoinitiated unimolecular decomposition of jet-cooled HNCO has been studied following S1(1A″)←S0(1A′) excitation near the thresholds of the spin-allowed dissociation channels: (1) H (2S)+NCO(X 2∏) and (2) NH(a 1Δ)+CO(X 1∑+), which are separated by 4470 cm-1. Photofragment yield spectra of NCO(X 2∏) and NH (a 1Δ) were obtained in selected regions in the 260-220 nm photolysis range. The NCO(X 2∏)yield rises abruptly at 38 380 cm-1 and the spectrum exhibits structures as narrow as 0.8 cm-1 near the threshold. The linewidths increase only slowly with photolysis energy. The jet-cooled absorption spectrum near the channel (1) threshold [D0(H+NCO)] was obtained using two-photon excitation via the S1 state, terminating in a fluorescent product. The absorption spectrum is similar to the NCO yield spectrum, and its intensity does not diminish noticeably above D0(H+NCO), indicating that dissociation near threshold is slow. The NCO product near threshold is cold, as is typical of a barrierless reaction. NH (a 1Δ) products appear first at 42 840 cm-1, but their yield is initially very small, as evidenced also by the insignificant decrease in the NCO yield in the threshold region of channel (2). The NH (a 1Δ) yield increases faster at higher photolysis energies and the linewidths increase as well. At the channel (2) threshold, the NH (a 1Δ) product is generated only in the lowest rotational level, J=2, and rotational excitation increases with photolysis energy. We propose that in the range 260-230 nm, HNCO (S1) undergoes radiationless decay terminating in S0/T1 followed by unimolecular reaction. Decompositions via channels (1) and (2) proceed without significant exit channel barriers. At wavelengths shorter than 230 nm, the participation of an additional, direct pathway cannot be ruled out. The jet-cooled photofragment yield spectra allow the determination, with good accuracy, of thermochemical values relevant to HNCO decomposition. The following heats of formation are recommended: ΔHf0(HNCO) = -27.8±0.4 kcal/mol, and ΔHf0(NCO)=30.3±0.4 kcal/mol. These results are in excellent agreement with recent determinations using different experimental techniques. © 1996 American Institute of Physics.
• Bieler, C. R., Sanov, A., & Reisler, H. (1995). Inelastic scattering of NO(²Π) with atomic and molecular colliders. Rotational and fine-structure excitations. Chemical Physics Letters, 235(3-4), 175-182.
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  Abstract: The inelastic collisions of NO (Trot ≤ 5 K) with Ar, Xe, CO, N2, O2, N2O and CO2 were studied in molecular beams at center-of-mass collision energies 750-2500 cm-1. Rotational, Λ-doublet and spin-orbit distributions of scattered NO(2Π 1 2 3 2) were determined. In all the scattering experiments (with the possible exception of Ar) no preferences wer observed in the Λ-doublet populations. The rotational distributions all appear Boltzmann-like, with somewhat different rotational temperatures. The populations of the 2Π 1 2 and 2Π 3 2 spin-orbit states depend on the nature of the collider, but do not show any clear relationship with the extent of rotational excitation. No evidence of long-range attractive interactions is revealed in any of the systems studied. © 1995.
• Jordan, ., Schatz, ., Troe, ., Luther, ., Lendvay, ., Sanov, A., Bieler, C. R., Capellos, C., Reisler, ., Moore, ., Smith, ., Flynn, ., Miller, W. H., Oref, ., Bradley, K. S., Clary, ., Child, ., Connor, ., Hynes, ., , Schinke, ., et al. (1995). General discussion. Faraday Discussions, 102, 451-483.
• Reid, S. A., Sanov, A., & Reisler, H. (1995). Resonances and fluctuations in the unimolecular reaction of NO₂. Faraday Discussions, 102, 129-146.
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  Abstract: Fluctuations and oscillations in the unimolecular reaction of NO2, and their manifestations in photofragment yield (PHOFRY) spectra, NO rotational state distributions and decomposition rates are examined. Comparisons between experimental and simulated PHOFRY spectra show that extraction of rates from linewidths in state-selected spectra is unjustified in the regime of overlapping resonances. Measurements of the alignment parameter of the NO product in the excess energy range EE = 200-500 cm-1 evidence the existence of fluctuations in the decay rate. Changes in the patterns of fluctuations and oscillations in the NO rotational state distributions reveal the progressive tightening of the transition state (TS) as the excess energy increases and the importance of exit-channel interactions beyond the TS. Distributions well fit on the average by phase-space calculations can be obtained even when the transition state is tight.
• Sanov, A., Bieler, C. R., & Reisler, H. (1995). Dissociation of highly excited NO₂ induced by collisions with Ar, CO, and O₂. Journal of Physical Chemistry, 99(19), 7339-7351.
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  Abstract: The gas-phase collision-induced dissociation (CID) of highly excited, mixed 2A1/2B2 states of NO2 with Ar, CO, and O2 is studied in crossed beams experiments with state-resolved detection of products. Both the internal and translational energy of the reactants are varied independently. The state-specific relative yield of the NO product as a function of the initial NO2 excitation is reported for excitation energies hv from dissociation threshold D0 to D0 - hv = 1000 cm-1. The relative collision energies are 750-2400 cm-1. The structure of the CID yield spectrum is similar to that observed in the fluorescence excitation spectrum of NO2, and the scaling of the collisional energy transfer efficiency can be described by an exponential decay law. NO is detected using laser ionization, and rotational and spin-orbit distributions are determined. CID is described fairly well by assuming a unimolecular decomposition (UMD) of NO2, collisionally excited to a range of excess energies above D0 with excitation probability determined by the exponential gap law. The average energy transferred per activating collision is in the range 110-310 cm-1, dependent on both the relative collision energy and the nature of the collider. Higher product spin-orbit excitations are observed with CO and O2 than with Ar which suggests a stronger exit-channel interaction with these colliders. © 1995 American Chemical Society.
• Sanov, A., Bieler, C. R., & Reisler, H. (1995). Fully quantum-state resolved study of NO₂ photodissociation. Correlated NO(²Π_Ω, ν = 0 J,A) + O(³P_j) distributions. Journal of physical chemistry, 99(37), 13637-13646.
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  Abstract: Relative O(3Pj = 2.1.0) spin-orbit populations correlated with specific NO[2ΠΩ = 1/2 , 3/2; ν = 0; f; Λ = Π(A′), Π(A″)] product states were obtained following photolysis of NO2 at excess energies E± = 390, 425, and 1054 cm-1. These fully quantum state-resolved measurements were carried out by recording spatial profiles of recoiling NO(2ΠΩ, J, Λ) products using polarized radiation for photolysis and state-selective laser ionization detection. The relative O(3Pj) populations correlated with each NO(2ΠΩ, J, Λ) state show marked fluctuations at each excess energy as a function of rotational state and Λ-doublet component. The relative populations also fluctuate as a function of excess energy. The O(3Pj) spin-orbit population ratios, when averaged over all measurements, exhibit distributions that are colder than statistical, in agreement with previous results. In particular, we find that, on average, O(3P1):O(3P2) population ratios correlated with the ground NO(2Π 1/2 ) state are colder than the corresponding ratios correlated with the excited NO(2Π3/2) spin-orbit state. These results are in agreement with the state-specific calculations of Katigiri and Kato [J. Chem. Phys. 1993, 99, 8805] and are discussed in terms of long-range nonadiabatic transitions among electronic states correlating asymptotically with different spin-orbit states of the ground NO(2Π) + O(3P) dissociation channel.
• Stolte, ., Delon, ., Schinke, ., Abel, ., Perry, ., Reisler, ., Troe, ., Hancock, ., Smith, ., Moore, ., Biesheuvel, ., Steege, T., Janssen, ., Bulthuis, ., Wardlaw, ., Herman, ., Rizzo, ., Mills, ., Quack, ., , Softley, ., et al. (1995). General discussion. Faraday Discussions, 102, 227-274.
• Bieler, C. R., Sanov, A., Hunter, M., & Reisler, H. (1994). Gas-phase collision-induced dissociation of highly excited NO₂. Journal of Physical Chemistry, 98(4), 1058-1060.
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  Abstract: The gas-phase collision-induced dissocation (CID) of highly excited NO2 with argon has been observed in a crossed beams experiment. The dissociation yield as a function of the initial NO2 excitation energy is reported. The structure of the CID signal is similar to that observed in the fluorescence excitation spectrum of NO2, and the scaling of energy transfer can be described by an exponential decay law. NO is detected state selectively using laser ionization, and state distributions are determined. © 1994 American Chemical Society.
• Sanov, A. M. (1992). Thermal diffusion effect on the relaxation rate of induced optical response of metal films of varying thickness excited by ultrashort laser pulses. Optics and Spectroscopy (English translation of Optika i Spektroskopiya), 73(5), 610-611.
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  Abstract: In this study the effect of thermal diffusion on the establishment of equilibrium of metal electrons with the lattice under irradiation of metal films of varying thickness of ultrashort laser pulses. Calculations are made of the relaxation time of the induced optical response as a function of film thickness.

Presentations

• Sanov, A. M. (2017, July). Photoelectron imaging spectroscopy of weakly bound anions and transient neutral molecules. Workshop on Advances in Theory of Electronic Resonances. Telluride, Colorado.
• Sanov, A. M. (2016, August). “Underemployed and looking for action: The plight of unpaired electrons in radicals and diradicals via anion photoelectron imaging” (invited talk). Conference on Advanced Particle Imaging Techniques: 1986-2016 and Beyond, Telluride, Colorado, August 7-12, 2016. Conference on Advanced Particle Imaging Techniques: 1986-2016 and Beyond, Telluride, Colorado, August 7-12, 2016..
• Sanov, A. M. (2016, July). "Underemployed and looking for action: The plight of unpaired electrons in diradicals, radicals, and radical anions" (invited talk). International Conference on Chemical Bonding, Kauai, Hawaii, July 14-18, 2016. Kauai, Hawaii.
• Sanov, A. M. (2016, November). “Quantum photography: Chemistry through the eyes of Schrödinger’s Cat”. Nazarbayev University, Astana, Kazakhstan, Department of Physics, 2 November 2016 (invited seminar).. Department of Physics, Nazarbayev University, Astana, Kazakhstan.
• Sanov, A. M. (2016, October). Underemployed and looking for action: The plight of unpaired electrons in reactive intermediates and other exotic species. University of Southern California, Los Angeles, 17 October 2016 (invited seminar).. Department of Chemistry, University of Southern California, Los Angeles..
• Sanov, A. M. (2016, October). “Underemployed and Looking for Action: The Plight of Unpaired Electrons in Radicals and Diradicals”. University of Arizona, Department of Physics, 28 October 2016 (invited seminar). University of Arizona, Department of Physics, Tucson.
• Sanov, A. (2015, December). Underemployed and looking for action: The plight of unpaired electrons in reactive intermediates and other elusive species. PACIFICHEM-2015 Congress. Honolulu, Hawaii.
• Sanov, A. (2015, June). Transient species and interstellar molecules via negative-ion photoelectron imaging spectroscopy. Wilhelm and Else Heraeus Seminar on Spectroscopy and applications of cold molecular ions. Bad Honnef (Germany): Wilhelm and Else Heraeus Foundation.
• Sanov, A. M. (2014, April). Photoelectron angular distributions in the photodetachment from mixed-character states. Gordon Research Conference on Molecular and Ionic Clusters. Lucca (Barga), Italy.
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  Dates: 04/27-05/02
• Sanov, A. M. (2014, February). Reactive Intermediates via Photoelectron Imaging Spectroscopy. Gordon Research Conference on Photoionization and Photodetachment.
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  Date: 02/23-02/28
• Sanov, A. M. (2014, February). Underemployed and Looking for Action: The Plight of Non-Bonding Electrons in Diradicals and Carbenes. Invited Departmental seminar. San Diego, CA: University of California.
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  Date: 02/13
• Sanov, A. M. (2014, September). Underemployed and Looking for Action: The Plight of Non-Bonding Electrons in Diradicals and Carbenes. Invited Departmental seminar. Albuquerque, NM: University of New Mexico.
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  Date: 09/12

Poster Presentations

• Sanov, A. M. (2014, Fall). Solvation effects on the angular distributions in cluster-anion photodetachment: The role of solute's electronic structure. Gordon Research Conference on Molecular and Ionic Clusters. Lucca (Barga), Italy.
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  Dates: 04/27-05/02
• Sanov, A. M. (2013, April). Chemistry through the eyes of Schrödinger’s Cat and other elusive creatures. Davis & Weed Symposium. NOT PROVIDED: University of Arizona.
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  Date: 04/12
• Sanov, A. M. (2013, Fall). Photoelectron angular distributions in anion photodetachment from mixed-character states. Wilhelm and Else Heraeus Seminar on High-Harmonic Spectroscopy. Bad Honnef, Germany.
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  Dates: 01/27-02/1
• Sanov, A. M. (2013, July). Radical and Diradical Reactive Intermediates via Anion Photoelectron Imaging. Conference on the Dynamics of Molecular Collisions. Lake Tahoe, California: Granlibakken.
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  Dates: 07/05-07/12
• Sanov, A. M. (2013, June). Photoelectron angular distributions in anion photodetachment from mixed-character states. XXV International Symposium on Molecular Beams. Prague, Czech Republic.
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  Dates: 06/09-06/14
• Sanov, A. M. (2013, March). Quantum photography: Snapshots of Chemistry, one electron at a time. NOT PROVIDED. NOT PROVIDED: Chemistry Club, Student Members of the American Chemical Society (SMACS), University of Arizona.
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  Date: 03/01
• Sanov, A. M. (2012, January). Solvation effects on photoelectron angular distributions: The roles of solvent and solute electronic structures. Western Spectroscopy Association Conference. Monterey, California: Asilomar Conference Center.
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  Dates: 01/25-01/27
• Sanov, A. M. (2012, May). Solvation effects on photoelectron angular distributions: The roles of solvent and solute electronic structures. NOT PROVIDED. New Mexico: Los Alamos National Laboratory.
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  Date: 05/03