Marissa L. Weichman

Slow photoelectron velocity-map imaging spectroscopy of cold negative ions

Anion slow photoelectron velocity-map imaging (SEVI) spectroscopy is a high-resolution variant of photoelectron spectroscopy used to study the electronic and geometric structure of atoms, molecules, and clusters. To benefit from the high resolution of SEVI when it is applied to molecular species, it is essential to reduce the internal temperature of the ions as much as possible. Here, we describe an experimental setup that combines a radio-frequency ion trap to store and cool ions with the high-resolution SEVI spectrometer. For C(5)(-), we demonstrate ion temperatures down to 10 ± 2 K after extraction from the trap, as measured by the relative populations of the two anion spin-orbit states. Vibrational hot bands and sequence bands are completely suppressed, and peak widths as narrow as 4 cm(-1) are seen due to cooling of the rotational degrees of freedom.

Spectroscopic observation of resonances in the F + H2 reaction

Glimpsing resonances as F and H2 react The reaction of fluorine atoms with hydrogen molecules has long provided a window into the subtle effects of quantum mechanics on chemical dynamics. Kim et al. now show that the system still has some secrets left to reveal. The authors applied photodetachment to FH2− anions and their deuterated analogs. This allowed them to intercept the reaction trajectory in the middle and thereby uncover unanticipated weakly bound resonances. Theoretical calculations explain these observations and predict additional similar features that have yet to be seen. Science, this issue p. 510 Combined theory and experiment uncover subtle weakly bound states along the pathway of a widely studied chemical reaction. Photodetachment spectroscopy of the FH2− and FD2− anions allows for the direct observation of reactive resonances in the benchmark reaction F + H2 → HF + H. Using cooled anion precursors and a high-resolution electron spectrometer, we observe several narrow peaks not seen in previous experiments. Theoretical calculations, based on a highly accurate F + H2 potential energy surface, convincingly assign these peaks to resonances associated with quasibound states in the HF + H and DF + D product arrangements and with a quasibound state in the transition state region of the F + H2 reaction. The calculations also reveal quasibound states in the reactant arrangement, which have yet to be resolved experimentally.

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.

Isomer-specific vibronic structure of the 9-, 1-, and 2-anthracenyl radicals via slow photoelectron velocity-map imaging.

Significance Polycyclic aromatic hydrocarbons (PAHs) are involved in soot nucleation following inefficient fuel combustion and are considered mutagens and environmental pollutants. They are also suspected to exist in the interstellar medium, although mechanisms for their formation in space are speculative. It is of great interest in these diverse fields to better characterize PAHs, including their dehydrogenated and charged derivatives, which are harder to isolate and probe. We use high-resolution anion photoelectron spectroscopy and quantum chemistry calculations to study the energetics, electronic states, and vibrational frequencies of the three dehydrogenated radical isomers of anthracene. These results provide signatures of these species for potential identification in space and illuminate subtle isomer-specific properties relevant to modeling their behavior in combustion and interstellar environments. Polycyclic aromatic hydrocarbons, in various charge and protonation states, are key compounds relevant to combustion chemistry and astrochemistry. Here, we probe the vibrational and electronic spectroscopy of gas-phase 9-, 1-, and 2-anthracenyl radicals (C14H9) by photodetachment of the corresponding cryogenically cooled anions via slow photoelectron velocity-map imaging (cryo-SEVI). The use of a newly designed velocity-map imaging lens in combination with ion cooling yields photoelectron spectra with <2 cm−1 resolution. Isomer selection of the anions is achieved using gas-phase synthesis techniques, resulting in observation and interpretation of detailed vibronic structure of the ground and lowest excited states for the three anthracenyl radical isomers. The ground-state bands yield electron affinities and vibrational frequencies for several Franck–Condon active modes of the 9-, 1-, and 2-anthracenyl radicals; term energies of the first excited states of these species are also measured. Spectra are interpreted through comparison with ab initio quantum chemistry calculations, Franck–Condon simulations, and calculations of threshold photodetachment cross sections and anisotropies. Experimental measures of the subtle differences in energetics and relative stabilities of these radical isomers are of interest from the perspective of fundamental physical organic chemistry and aid in understanding their behavior and reactivity in interstellar and combustion environments. Additionally, spectroscopic characterization of these species in the laboratory is essential for their potential identification in astrochemical data.

Structural Isomers of Ti2O4 and Zr2O4 Anions Identified by Slow Photoelectron Velocity-Map Imaging Spectroscopy

High-resolution anion photoelectron spectra are reported for the group 4 metal dioxide clusters Ti2O4(-) and Zr2O4(-). Slow photoelectron velocity-map imaging (SEVI) spectroscopy of cryogenically cooled, mass-selected anions yields photoelectron spectra with submillielectronvolt resolution, revealing extensive and well-resolved vibrational progressions. By comparison of the spectra with Franck-Condon simulations, we have identified the C(2v) and C(3v) isomers as the ground states of Ti2O4(-) and Zr2O4(-) anions, respectively. Minor contributions from the C(2h) isomer of Ti2O4(-) and the C(2v) isomer of Zr2O4(-) are also seen. The SEVI spectra yield upper bounds for the adiabatic detachment energies, as well as vibrational frequencies for various modes of the neutral Ti2O4 and Zr2O4 species.

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.

Feshbach resonances in the exit channel of the F + CH3OH → HF + CH3O reaction observed using transition-state spectroscopy

The transition state governs how chemical bonds form and cleave during a chemical reaction and its direct characterization is a long-standing challenge in physical chemistry. Transition state spectroscopy experiments based on negative-ion photodetachment provide a direct probe of the vibrational structure and metastable resonances that are characteristic of the reactive surface. Dynamical resonances are extremely sensitive to the topography of the reactive surface and provide an exceptional point of comparison with theory. Here we study the seven-atom F + CH3OH → HF + CH3O reaction using slow photoelectron velocity-map imaging spectroscopy of cryocooled CH3OHF− anions. These measurements reveal spectral features associated with a manifold of vibrational Feshbach resonances and bound states supported by the post-transition state potential well. Quantum dynamical calculations yield excellent agreement with the experimental results, allow the assignment of spectral structure and demonstrate that the key dynamics of complex bimolecular reactions can be captured with a relatively simple theoretical framework. The transition state governs how bonds form and cleave during a reaction — its direct characterization is a long-standing challenge. Now, the F + CH3OH → HF + CH3O reactive surface has been studied using photoelectron velocity-map imaging spectroscopy of cryo-cooled anions, revealing vibrational Feshbach resonances and bound states supported by the post-transition-state potential well. The experiments agree well with quantum dynamical calculations.

Vibrational and electronic structure of the α- and β-naphthyl radicals via slow photoelectron velocity-map imaging.

Slow photoelectron velocity-map imaging (SEVI) spectroscopy has been used to study the vibronic structure of gas-phase α- and β-naphthyl radicals (C(10)H(7)). SEVI of cryogenically cooled anions yields spectra with <4 cm(-1) resolution, allowing for the observation and interpretation of congested vibrational structure. Isomer-specific photoelectron spectra of detachment to the radical ground electronic states show detailed structure, allowing assignment of vibrational fundamental frequencies. Transitions to the first excited states of both radical isomers are also observed; vibronic coupling and photodetachment threshold effects are considered to explain the structure of the excited bands.

Encoding of vinylidene isomerization in its anion photoelectron spectrum

The quantum mechanics of a hydrogen hop Hydrogen migration between adjacent carbons is widespread in the reaction mechanisms of organic chemistry. DeVine et al. used photoelectron spectroscopy to discern the quantum mechanical underpinnings of this 1,2 shift in a prototypical case: conversion of vinylidene (H2CC) to acetylene (HCCH). The technique probed specific states of vinylidene by ejecting electrons with varying energies from a negative ion precursor. Experimental data and accompanying theoretical simulations pinpointed a vibrational rocking mode that facilitated the migration. Replacement of hydrogen with its heavier deuterium isotope disrupted this pathway. Science, this issue p. 336 Photoelectron spectroscopy reveals the quantum mechanical underpinnings of a 1,2-hydrogen shift reaction. Vinylidene-acetylene isomerization is the prototypical example of a 1,2-hydrogen shift, one of the most important classes of isomerization reactions in organic chemistry. This reaction was investigated with quantum state specificity by high-resolution photoelectron spectroscopy of the vinylidene anions H2CCˉ and D2CCˉ and quantum dynamics calculations. Peaks in the photoelectron spectra are considerably narrower than in previous work and reveal subtleties in the isomerization dynamics of neutral vinylidene, as well as vibronic coupling with an excited state of vinylidene. Comparison with theory permits assignment of most spectral features to eigenstates dominated by vinylidene character. However, excitation of the ν6 in-plane rocking mode in H2CC results in appreciable tunneling-facilitated mixing with highly vibrationally excited states of acetylene, leading to broadening and/or spectral fine structure that is largely suppressed for analogous vibrational levels of D2CC.

Rovibronic structure in slow photoelectron velocity-map imaging spectroscopy of CH2CN− and CD2CN−

We report high-resolution anion photoelectron spectra of the cryogenically cooled cyanomethide anion, CH2CN(-), and its isotopologue, CD2CN(-), using slow photoelectron velocity-map imaging (SEVI) spectroscopy. Electron affinities of 12 468(2) cm(-1) for CH2CN and 12 402(2) cm(-1) for CD2CN are obtained, demonstrating greater precision than previous experiments. New vibrational structure is resolved for both neutral species, especially activity of the ν5 hydrogen umbrella modes. The ν6 out-of-plane bending mode fundamental frequency is measured for the first time in both systems and found to be 420(10) cm(-1) for CH2CN and 389(8) cm(-1) for CD2CN. Some rotational structure is resolved, allowing for accurate extraction of vibrational frequencies. Temperature-dependent SEVI spectra show marked effects ascribed to controlled population of low-lying anion vibrational levels. We directly measure the inversion splitting between the first two vibrational levels of the anion ν5 umbrella mode in both species, finding a splitting of 130(20) cm(-1) for CH2CN(-) and 81(20) cm(-1) for CD2CN(-). Franck-Condon forbidden activity is observed and attributed to mode-specific vibrational autodetachment from the CH2CN(-) and CD2CN(-) dipole bound excited states. We also refine the binding energy of the anion dipole bound states to 39 and 42 cm(-1), respectively, for CH2CN(-) and CD2CN(-).