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 (15)A2←(16)B2 photodetachment transition of Fe4O(-) and the (17)A'←(18)A″ 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 (15)A2 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.