High-resolution photoelectron imaging spectroscopy of cryogenically cooled Fe₄O^- and Fe₅O^-

We report high-resolution photodetachment spectra of the cryogenically cooled iron monoxide clusters Fe₄O^- and Fe₅O^- obtained with slow photoelectron velocity-map imaging (cryo-SEVI). Well-resolved vibrational progressions are observed in both sets of spectra, and transitions to low-lying excited states of both species are seen. In order to identify the structural isomers, electronic states, and vibrational modes that contribute to the cryo-SEVI spectra of these clusters, experimental results are compared with density functional theory calculations and Franck-Condon simulations. The main bands observed in the SEVI spectra are assigned to the ¹⁵A₂→¹⁶B₂ photodetachment transition of Fe₄O^- and the ¹⁷A′→¹⁸A″ photodetachment transition of Fe₅O^-. We report electron affinities of 1.6980(3) eV for Fe₄O and 1.8616(3) eV for Fe₅O, although there is some uncertainty as to whether the ¹⁵A₂ state is the true ground state of Fe₄O. The iron atoms have a distorted tetrahedral geometry in Fe₄O^0/- and a distorted trigonal-bipyramidal arrangement in Fe₅O^0/-. For both neutral and anionic species, the oxygen atom preferably binds in a μ₂-oxo configuration along the cluster edge. This finding is in contrast to prior predictions that Fe₅O^0/- exhibits a μ₃ face-bound structure.