Slow electron velocity-map imaging of cold and vibrationally excited anions as a probe of spectroscopy and dynamics

Slow-electron velocity-map imaging of cryogenically-cooled anions (cryo-SEVI) has been developed over the last several years in our laboratory. This experiment is a high resolution variant of anion photoelectron spectroscopy and yields well-resolved photoelectron spectra for polyatomic molecular and cluster anions for which more conventional methods show little or no resolved vibrational structure. Cryo-SEVI has been shown to be a powerful spectroscopic probe of bare and complexed metal oxide clusters that serve as model systems for a range of catalytic reactions. In addition, it has been applied to the transition state spectroscopy of benchmark bimolecular and unimolecular chemical reactions. We have recently incorporated a tunable infrared laser into this experiment that can vibrationally excite anions prior to photodetachment. This new implementation, IR cryo-SEVI, enables one to access neutral vibrational levels that cannot be reached from the anion ground state. It also provides a novel probe of anharmonic coupling in both the anion and neutral. Moreover, IR cryo-SEVI provides a means of obtaining anion vibrational frequencies in the gas phase without rare-gas tagging. Hence, both anion and neutral vibrational frequencies can be obtained from a single experimental protocol. Examples will be presented for OHˉ, NO₃ˉ CH₂CHOˉ (vinoxide), and H₂CCˉ (vinylidene). Vibrational pre-excitation of the vinylidene anion is of particular interest as one can access vibrational levels of H₂CC that promote isomerization to the much deeper HCCH well.