Slow Electron Velocity-map Imaging Research Articles

Slow photoelectron velocity-map imaging spectroscopy of the Fe₃O⁻ and Co₃O⁻ anions.

We report high-resolution photoelectron spectra of the transition metal suboxide clusters Fe3O(-) and Co3O(-). The combination of slow electron velocity-map imaging and cryogenic cooling yields vibrationally well-resolved spectra, from which we obtain precise values of 1.4408(3) and 1.3951(4) eV for the electron affinities of Fe3O and Co3O. Several vibrational frequencies of the neutral ground state Fe3O and Co3O clusters are assigned for the first time, and a low-lying excited state of Fe3O is observed. The experimental results are compared with density functional electronic structure calculations and Franck-Condon spectral simulations, enabling identification of the structural isomer and electronic states. As has been found in photoelectron spectra of other trimetal oxo species, Fe3O(0/-) and Co3O(0/-) are assigned to a μ2-oxo isomer with planar C(2v) symmetry. We identify the ground states of Fe3O(-) and Co3O(-) as (12)A1 and (9)B2 states, respectively. From these states we observe photodetachment to the (11)B2 ground and (13)A1 excited states of Fe3O, as well as to the (8)A1 ground state of Co3O.

Elucidating the Electronic Structures of the Ground States of the VO2(-/0) Clusters: Synergism between Computation and Experiment.

Electronic structures of VO2 and its anion were investigated with density functional theory (DFT), complete active space second-order perturbation theory (CASPT2), and restricted coupled-cluster with single, double, and perturbative triple excitations (RCCSD(T)) computational quantum chemical methods. The results show that there is a near-degeneracy of the lowest (3)B1, (3)A1, and (1)A1 states of the anion. Therefore, the 532 and 193 nm photoelectron spectra of VO2(-) are interpreted by exploring these states as possible initial states. The anionic ground state was identified at the highest computational level, that is, RCCSD(T), as (3)B1 allowing the X band to be assigned to the (3)B1 → (2)B1 transition, while the lower intensity and lower binding energy X' and X″ features are ascribed to the (1)A1 → (2)A1 and (3)A1 → (2)A1 ionizations, respectively. The latter assignment is different from the recently proposed assignment of the corresponding slow electron velocity-map imaging (SEVI) spectra. Further, the A band is suggested to be mainly the result of an ionization from (3)B1 to 2(2)A1. For all these ionizations an electron is removed from a predominant metal orbital. The higher energy bands B and C on the contrary can be ascribed as electron detachments out of molecular orbitals largely located on the oxygen centers. More precisely, the B band is attributed to the ionizations from (3)B1 to (4)A2 and (2)A2, while the C band is proposed to originate from the (3)B1 → (4)B1 and (3)B1 → 2(2)B1 ionizations. The proposed novel assignment is further corroborated by calculating the Franck-Condon factors, which largely agree with the experimental vibrational progressions of the SEVI spectra.

The 700-1500 cm−1 region of the S1 ($\widetilde A{}^1B_2$Ã1B2) state of toluene studied with resonance-enhanced multiphoton ionization (REMPI), zero-kinetic-energy (ZEKE) spectroscopy, and time-resolved slow-electron velocity-map imaging (tr-SEVI) spectroscopy

We report (nanosecond) resonance-enhanced multiphoton ionization (REMPI), (nanosecond) zero-kinetic-energy (ZEKE) and (picosecond) time-resolved slow-electron velocity map imaging (tr-SEVI) spectra of fully hydrogenated toluene (Tol-h8) and the deuterated-methyl group isotopologue (α3-Tol-d3). Vibrational assignments are made making use of the activity observed in the ZEKE and tr-SEVI spectra, together with the results from quantum chemical and previous experimental results. Here, we examine the 700-1500 cm(-1) region of the REMPI spectrum, extending our previous work on the region ≤700 cm(-1). We provide assignments for the majority of the S1 and cation bands observed, and in particular we gain insight regarding a number of regions where vibrations are coupled via Fermi resonance. We also gain insight into intramolecular vibrational redistribution in this molecule.

Spectroscopic Separation of the Methyl Internal-Rotational Isomers of Thioanisole Isotopomers (C6H5S-CH2D and C6H5S-CHD2)

Two distinct rotational isomers of thioanisole-d1 (C6H5S-CH2D) and thioanisole-d2 (C6H5S-CHD2) with respect to the internal rotation of the methyl moiety have been identified and characterized spectroscopically using the resonantly enhanced two photon ionization, UV-UV hole burning, and slow-electron velocity map imaging techniques. From the statistical weights, the definite assignment for the specific rotational isomer of each isotopomer has been successfully done, providing isomer-specific ionization energies and vibrational frequencies of S1/D0 states. Detailed molecular structures, the methyl internal-rotor barrier, and normal-mode descriptions for selective vibrations are discussed with the aid of density functional theory calculations.

Vibronic structure of VO2 probed by slow photoelectron velocity-map imaging spectroscopy.

We report high-resolution anion photoelectron spectra of vanadium dioxide (VO2 (-)) obtained by slow electron velocity-map imaging of trapped and cryogenically cooled anions. Vibrationally resolved spectra are obtained for photodetachment to the first three neutral electronic states, giving an electron affinity of 1.8357(5) eV for the X̃A12 ground state and term energies of 0.1845(8) eV and 0.8130(5) eV for the ÃB12 and B̃A12 excited states, respectively. The vibrational fundamentals ν1 and ν2 are obtained for all three states. Experimental assignments are confirmed by energies from electronic structure calculations and Franck-Condon spectral simulations. These simulations support assigning the anion ground state as the X̃B13 state. With this assignment, photodetachment to the B̃A12 state involves a nominally forbidden two-electron transition, suggesting extensive configuration interaction in neutral VO2.

Resonances in the Entrance Channel of the Elementary Chemical Reaction of Fluorine and Methane

Extending the fully quantum-state-resolved description of elementary chemical reactions beyond three or four atom systems is a crucial issue in fundamental chemical research. Reactions of methane with F, Cl, H or O are key examples that have been studied prominently. In particular, reactive resonances and nonintuitive mode-selective chemistry have been reported in experimental studies for the F+CH4 →HF+CH3 reaction. By investigating this reaction using transition-state spectroscopy, this joint theoretical and experimental study provides a clear picture of resonances in the F+CH4 system. This picture is deduced from high-resolution slow electron velocity-map imaging (SEVI) spectra and accurate full-dimensional (12D) quantum dynamics simulations in the picosecond regime.

Slow photoelectron velocity-map imaging spectroscopy of the C9H7 (indenyl) and C13H9 (fluorenyl) anions

High-resolution photoelectron spectra are reported of the cryogenically cooled indenyl and fluorenyl anions, C9H7(-) and C13H9(-), obtained with slow electron velocity-map imaging. The spectra show well-resolved transitions to the neutral ground states, giving electron affinities of 1.8019(6) eV for indenyl and 1.8751(3) eV for fluorenyl. Numerous vibrations are observed and assigned for the first time in the radical ground states, including several transitions that are allowed only through vibronic coupling. The fluorenyl spectra can be interpreted with a Franck-Condon simulation, but explaining the indenyl spectra requires careful consideration of vibronic coupling and photodetachment threshold effects. Comparison of high- and low-resolution spectra along with measurements of photoelectron angular distributions provide further insights into the interplay between vibronic coupling and the photodetachment dynamics. Transitions to the neutral first excited states are also seen, with term energies of 0.95(5) eV and 1.257(4) eV for indenyl and fluorenyl, respectively. Those peaks are much wider than the experimental resolution, suggesting that nearby conical intersections must be considered to fully understand the vibronic structure of the neutral radicals.

Slow Photoelectron Velocity-Map Imaging Spectroscopy of Cold Thiozonide (S3–)

We report high-resolution anion photoelectron spectra of thiozonide (S3(-)) acquired by slow electron velocity-map imaging (SEVI). The ions were cryogenically cooled within an ion trap before photodetachment. We measure an electron affinity of 2.3630(9) eV, resolving discrepancies in previously reported photoelectron spectra that resulted from the presence of vibrational hot bands. The SEVI spectrum shows well-resolved, extended vibrational progressions in the symmetric stretch and bending modes of S3, yielding accurate frequencies for both.

Spectroscopic study on nonradiative transition and ionization of 5-methylpyrimidine at S1 probed by the slow-electron velocity-map imaging (SEVI) technique

Slow electron velocity-map imaging (SEVI) has been employed for the ionization spectroscopic study of the jet-cooled 5-methylpyrimidine in the first singlet 1(nπ∗) excited state. Resonant-enhanced two-color two-photon (1+1′) ionization spectrum gives well-resolved S1 vibrational structures up to ∼2400cm−1 above the origin whereas spectral bands becomes completely broadened in the higher energy region as coupling of individual S1 bands to dark states becomes significant. Ionization of the S1 5-methylpyrimidine with different ionization wavelengths at several different delay times reveals the property of low-lying 3(ππ∗) triplet dark state which is prepared by fast nonadiabatic transition. SEVI spectra taken via various S1 intermediate states provide the detailed vibrational structures of the 5-methylpyrimidine cation (D0), also giving the adiabatic ionization energy of 9.1052±0.0030eV (73438±24cm−1). Geometrical changes in the S0–S1–D0 transitions are discussed from Franck-Condon simulations based on calculations by (time-dependent) density functional theory.

High-resolution anion photoelectron spectra of TiO2−, ZrO2−, and HfO2− obtained by slow electron velocity-map imaging

High-resolution anion photoelectron spectra of the Group 4 metal dioxides TiO2(-), ZrO2(-), and HfO2(-) are reported, using slow electron velocity-map imaging (SEVI) combined with ion trapping and cryogenic cooling. The resulting spectra exhibit sub-meV resolution with no congestion from hot bands. Electron affinities are obtained with greater precision than in previous photodetachment experiments, with values of 1.5892(5) eV, 1.6397(5) eV, and 2.1045(5) eV, for TiO2, ZrO2, and HfO2, respectively. We obtain precise values for all of the vibrational frequencies of the neutral X[combining tilde](1)A1 ground states, except for the ν3 mode of HfO2. Weak activity observed in the forbidden ν3 mode for TiO2 and ZrO2 is attributed to Herzberg-Teller coupling to the Ã(2)B2 excited state.

Vibrationally resolved transition state spectroscopy of the F + H2 and F + CH4 reactions

The transition state regions of the F + para-H2, F + normal-H2, F + CH4 and F + CD4 reactions have been studied by slow electron velocity-map imaging (SEVI) spectroscopy of the anionic precursor clusters para-FH2-, normal-FH2-, FCH4 and FCD4. The F + H2 results improve on previously published photoelectron spectra, resolving a narrow peak that appears in the same position in the para-FH2 and normal-FH2- spectra, and suggesting that additional theoretical treatment is necessary to fully describe and assign the experimental results. A small peak in the para-FH2- results is also identified, matching simulations of a product resonance in the v' = 3 vibrational level. SEVI spectra of the 2P3/2 bands of FCH4- and FCD4- show extended structure from transitions to the entrance valley van der Waals region and the reactant side of the F + CH4 transition state region. Much of this structure is attributed to bending or hindered rotation of the methane moiety and may be a spectroscopic signature of reactive resonances.

Study of RgS− and RgS (Rg = Ne, Ar, and Kr) via slow photoelectron velocity-map imaging spectroscopy and ab initio calculations

High-resolution photoelectron spectra of RgS(-) (Rg = Ne, Ar, and Kr) were obtained using slow electron velocity-map imaging (SEVI). The SEVI spectra reveal well-resolved vibrational transitions between multiple spin-orbit states of RgS(-) and RgS, both of which are open-shell species. Detailed assignments are made by comparison with theoretical simulations based on high level ab initio calculations and a atoms-in-molecule model that accounts for spin-orbit coupling in the anion and neutral. Several RgS(-) and RgS vibrational frequencies and excited-state term energies are accurately determined from the analysis of the experimental spectra and are found to be in excellent agreement with the calculated values.

Slow photoelectron velocity-map imaging spectroscopy of the n-methylvinoxide anion

High resolution photoelectron spectra of the n-methylvinoxide anion and its deuterated isotopologue are obtained by slow electron velocity-map imaging. Transitions between the X̃(1)A' anion ground electronic state and the radical X̃(2)A" and Ã(2)A' states are observed. The major features in the spectra are attributed to transitions involving the lower energy cis conformers of the anion and neutral, while the higher energy trans conformers contribute only a single small peak. Franck-Condon simulations of the X̃(2)A" ← X̃(1)A' and Ã(2)A' ← X̃(1)A' transitions are performed to assign vibrational structure in the spectrum and to aid in identifying peaks in the cis-n-methylvinoxy X̃ (2)A" band that occur only through vibronic coupling. The experimental electron affinity and à state term energy are found to be EA = 1.6106 ± 0.0008 eV and T(0) = 1.167 ± 0.002 eV for cis-n-methylvinoxy.

Slow photoelectron velocity-map imaging spectroscopy of the phenoxide and thiophenoxide anions

High resolution anion photodetachment spectra of the phenoxide and thiophenoxide anions were obtained with slow electron velocity-map imaging. The spectra show transitions to the X(2)B(1) neutral states of both species and to the Ã(2)B(2) state of the thiophenoxy radical. Comparison of the spectra with Franck-Condon simulations allows several gas-phase vibrations to be assigned. The adiabatic electron affinities are determined to be 2.2538(8) eV and 2.3542(6) eV for phenoxy and thiophenoxy, respectively. The term energy of the Ã(2)B(2) state of thiophenoxy is found to be 0.3719(9) eV, higher than the values reported in photodissociation experiments of thiophenol.

Vibronic Analysis of the S1−S0 Transition of Phenylacetylene Using Photoelectron Imaging and Spectral Intensities Derived from Electronic Structure Calculations

The vibrational structure of the S(1)-S(0) electronic band of phenylacetylene has been recorded by 1 + 1 resonance-enhanced multiphoton ionization, accompanied by slow electron velocity map imaging photoelectron spectroscopy at each resonant vibrational band. Assignments of the S(1) vibrations (up to 2000 cm(-1) above the band origin) are based upon the relative intensities of the vibronic bands calculated by complete second-order vibronic coupling, vibration-rotation (Coriolis and Birss) coupling calculations, and the vibrational structure of the S(1) resonant photoelectron spectra. Although this is an allowed electronic transition, the relative intensities of the a(1) bands are often largely determined by vibronic coupling rather than simple Franck-Condon factors, and second-order coupling is substantial. Nonsymmetric vibrations have intensities obtained through either vibronic or Coriolis coupling, and the calculations have been instrumental in discriminating between alternate possibilities in the assignments. Strong vibronic effects are expected to be present in the spectra of most monosubstituted benzenes, and the calculations presented here show that theoretical treatments based upon electronic structure calculations will generally be useful in the analysis of their spectra.

Slow Photoelectron Velocity-Map Imaging of the i-Methylvinoxide Anion

High-resolution photoelectron spectra of the i-methylvinoxide anion are obtained by slow electron velocity-map imaging. The transitions between the anion ground electronic state, X̃ (1)A', and the radical X̃ (2)A'', and à (2)A' states are measured. Franck-Condon simulations of the X̃ (2)A'' ← X̃ (1)A' transition are performed to aid in assigning peaks, yielding several vibrational frequencies for the first time. Additional structure in the spectrum of the X̃ (2)A'' ← X̃ (1)A' transition is attributed to hindered rotor motion of the methyl group. Simulation of methyl torsional states allows identification of the true origin peak, yielding the experimental adiabatic electron affinity, EA = 1.747 ± 0.002 eV, and the à state term energy, T(0) = 1.037 ± 0.002 eV. Comparison with the fully deuterated isotopologue further supports the assignment of hindered rotor progressions. The favored methyl rotor position is eclipsed in the radical X̃ state and staggered in the anion X̃ and radical à states.

Slow photoelectron velocity-map imaging of the CnH− (n = 5–9) anions

High-resolution photoelectron spectra of the CnH− anions with n = 5–9 are acquired with slow electron velocity-map imaging (SEVI). Spectral features are assigned with the help of electronic structure calculations and Franck–Condon simulations. Well-resolved transitions to the linear 2Π and ã4Σ− neutral states are observed for species with an odd number of carbon atoms. For C6H− and C8H−, transitions to the 2Π neutral ground state and the low lying Ã2Σ+ excited state are observed. Precise electron affinities, term energies, fine structure splittings, and gas-phase vibrational frequencies are determined. The C5H−, C7H− and C9H− SEVI spectra are consistent with the anions having 3Σ− linear triplet ground states.

Anion Photoelectron Spectroscopy of C3N− and C5N−

Anion photoelectron spectroscopy of C(3)N(-) and C(5)N(-) is performed using slow electron velocity-map imaging (SEVI) and field-free time-of-flight (TOF), respectively. The SEVI spectrum exhibits well-resolved vibrational transitions from the linear C(3)N(-) ground state to the corresponding C(3)N ground state. The TOF spectrum comprises transitions arising from the linear C(5)N(-) ground state to the corresponding neutral ground and excited states. This study yields the adiabatic electron affinities of C(3)N and C(5)N to be 4.305 +/- 0.001 and 4.45 +/- 0.03 eV, respectively, and a term value of 560 +/- 120 cm(-1) for the A(2)Pi state of C(5)N. Vibrational frequencies for the degenerate cis and trans bending modes of C(3)N(-) are also extracted.

The photoelectron spectrum of the ethoxide anion: Conical intersections, the spin-orbit interaction, and sequence bands

The negative ion photoelectron spectrum of the ethoxide anion (ethoxide-h(5)) and that of its fully deuterated analog, ethoxide-d(5), are calculated using the multimode vibronic coupling approach. A two state quasidiabatic Hamiltonian H(d) is constructed which includes all terms through second order in the full 18 dimensional internal coordinate space. H(d) is centered at the ab initio determined minimum energy crossing (MEX) point on the symmetry-allowed (2)A(")-(2)A(') accidental seam of conical intersection and determined from ab initio energy gradients and derivative couplings. It reproduces the local topography of the (2)A(")-(2)A(') MEX, in addition to accurately representing the geometries, energetics, and harmonic frequencies of equilibrium and saddle point structures located on the ground electronic state potential energy surface in the vicinity of the MEX. Spin-orbit effects are included. The results for ethoxy-h(5) are compared to photoelectron and slow electron velocity-map imaging (SEVI) spectra. By comparing the measured and predicted photoelectron spectrum, the accuracy of the electronic structure treatment is inferred. The existence of sequence bands in the SEVI spectrum is established.

Study of KrO− and KrO via Slow Photoelectron Velocity-Map Imaging Spectroscopy and ab Initio Calculations

The high-resolution photoelectron spectrum of KrO(-) was obtained using slow electron velocity-map imaging (SEVI). The SEVI spectrum reveals numerous vibronic transitions between multiple electronic states of KrO(-) and KrO, both of which are open-shell species. Detailed assignments are made by comparison with theoretical simulations based on high level ab initio calculations and an atoms-in-molecule model that accounts for spin-orbit coupling in the anion and neutral. Several KrO(-) and KrO vibrational frequencies and excited-state term energies are accurately determined from the analysis of the experimental spectra and are found to be in good agreement with the calculated values.