Slow photoelectron velocity-map imaging spectroscopy of the Fe₃O^- and Co₃O^- anions

We report high-resolution photoelectron spectra of the transition metal suboxide clusters Fe₃O^- and Co₃O^-_. The combination of slow electron velocity-map imaging and cryogenic cooling yields vibrationally well-resolved spectra, from which we obtain precise values of 1.4408(3) and 1.3951(4) eV for the electron affinities of Fe₃O and Co₃O. Several vibrational frequencies of the neutral ground state Fe₃O and Co₃O clusters are assigned for the first time, and a low-lying excited state of Fe₃O is observed. The experimental results are compared with density functional electronic structure calculations and Franck-Condon spectral simulations, enabling identification of the structural isomer and electronic states. As has been found in photoelectron spectra of other trimetal oxo species, Fe₃O^0/- and Co₃O^0/- are assigned to a μ₂-oxo isomer with planar C_2v symmetry. We identify the ground states of Fe₃O^- and Co₃O^- as ¹²A₁ and ⁹B₂ states, respectively. From these states we observe photodetachment to the ¹¹B₂ ground and ¹³A₁ excited states of Fe₃O, as well as to the ⁸A₁ ground state of Co₃O.