High Resolution Photoelectron Spectroscopy of the Acetyl Anion

Isomer-Dependent Electron Affinities of Fluorophenyl Radicals, •C6H5-xFx (2 ≤ x ≤ 4)

Di-, tri-, and tetrafluorophenyl radicals each have three regioisomers, several of which can form multiple distinct radical structures. We present the photoelectron spectra of the di-, tri-, and tetrafluorophenide regioisomer anions generated from their associated fluorobenzene precursors. By comparing the spectra to the results of density functional theory calculations, we determine that in cases where more than one possible radical isomer is possible for a given regioisomer (radicals formed from 1,2-difluorobenzene, 1,3-difluorobenzene, 1,2,3-trifluorobenzene, and 1,2,4-trifluorobenzene) the most stable anion corresponds to a less stable neutral, suggesting that the reactive C-center on these fluorine-substituted phenyl groups can be controlled by charge state. Full analyses of the spectra and computational results yield further insights into the differences between the electronic and molecular structures of the fluorophenyl radicals and their associated anions.

Vibrational wave-packet dynamics of the silver pentamer probed by femtosecond NeNePo spectroscopy

Vibrational wave-packet dynamics on the ground electronic state of the neutral silver pentamer (Ag5) are studied by femtosecond (fs) pump-probe spectroscopy using the 'negative ion - to neutral - to positive ion' (NeNePo) excitation scheme. A vibrational wave packet is prepared on the 2A1 state of Ag5via photodetachment of mass-selected, cryogenically cooled Ag5- anions using a fs pump pulse. The temporal evolution of the vibrational wave packet is then probed by an ultrafast probe pulse via resonant multiphoton ionization to Ag5+. Frequency analysis of the fs NeNePo transients for pump-probe delay times from 0.2 to 8 ps reveals three primary beating frequencies at 157 cm-1, 101 cm-1 and 56 cm-1 as well as four weaker features. A comparison of these experimentally obtained beating frequencies to harmonic normal mode frequencies calculated from electronic structure calculations confirms that Ag5 in the gas phase adopts a planar trapezoidal geometry, similar to that previously observed in solid argon. The dependence of the ionization yield on the laser polarization indicates a s-d wave electron photodetachment from a 'p-type' occupied molecular orbital of Ag5. Franck-Condon analysis shows that both processes, photodetachment and subsequent photoionization determine the beating frequencies probed in the time-dependent cation yield. The present study extends the applicability of fs NeNePo spectroscopy to characterize the vibrational spectra in the far-IR frequency range in the absence of perturbations from a medium or a messenger atom to mass-selected neutral metal clusters with more than three atoms in the ground electronic states.

Trend in the Electron Affinities of Fluorophenyl Radicals ·C6H5-xFx (1 ≤ x ≤ 4)

The electron affinities (EAs) of a series of ·C6H5-xFx (1 ≤ x ≤ 4) fluorophenyl radicals are determined from the photoelectron spectra of their associated fluorophenide anions generated from C6H6-xFx (1 ≤ x ≤ 4) fluorobenzene precursors. The spectra show a near-linear incremental increase in EA of 0.4 eV/x. The spectra exhibit vibrationally unresolved and broad detachment transitions consistent with significant differences in the molecular structures of the anion and neutral radical species. The experimental EAs and broad spectra are consistent with density functional theory calculations on these species. While the anion detachment transitions all involve an electron in a non-bonding orbital, the differences in structure between the neutral and anion are in part due to repulsion between the lone pair on the C-center on which the excess charge is localized and neighboring F atoms. The C6H5-xFx- (2 ≤ x ≤ 4) spectra show features at lower binding energy that appear to be due to constitutional isomers formed in the ion source.

Spectroscopy of Radicals, Clusters, and Transition States Using Slow Electron Velocity-Map Imaging of Cryogenically Cooled Anions

Slow electron velocity-map imaging of cryogenically cooled anions (cryo-SEVI) is a high-resolution variant of anion photoelectron spectroscopy that has been applied with considerable success over the years to the study of radicals, size-selected clusters, and transition states for unimolecular and bimolecular reactions. Cryo-SEVI retains the versatility of conventional anion photoelectron spectroscopy while offering sub-meV resolution, thereby enabling the resolution of vibrational structure in the photoelectron spectra of complex anions. This Feature Article describes recent experiments in our laboratory using cryo-SEVI, including a new research direction in which anions are vibrationally pre-excited with an infrared laser pulse prior to photodetachment.