A new proposal for the ground state of the FeO- cluster in the gas phase and for the assignment of its photoelectron spectra.

High-level multireference CASPT2 calculations are performed to determine the electronic structures of the FeO and FeO(-) clusters. Geometry optimizations of all possible low-lying states of the two unsaturated complexes are carried out at the mentioned theoretical level by calculating their potential energy curves. FeO(-) is proposed to possess a (6)Sigma(+) ground state as opposed to the (4)Delta state, which has been put forward as the ground state by previous computational studies. On this basis an alternative assignment of the photoelectron spectra of this anion is postulated. All features of these spectra are tentatively attributed to either septet or quintet states of the neutral FeO cluster. The lowest energy band in the spectra is then the result of electron detachments to the (5)Delta ground state and the (5)Sigma(+) lowest excited state. The second lowest band might be due to the (7)Sigma(+). Two higher energy bands are thought to originate from two (5)Pi states and a (5)Phi. Vibrational progressions that are observed for some bands could be explained in terms of the calculated potential energy profiles of the relevant states. Credibility for the proposed assignment is rendered by the good correspondence between experiment and our computational method concerning the bond distance and dipole moment for the ground state of FeO.