Ce in the+4 oxidation state: Anion photoelectron spectroscopy and photodissociation of small Ce$_x$O$_y$H$_z$$^-$ molecules

Abstract

The anion photoelectron (PE) spectra of a range of small mono-cerium molecular species, along with the Ce$_2$O$_4$$^−$ and Ce$_3$O$_6$− stoichiometric clusters, are presented and analyzed with the support of density functional theory calculations. A common attribute of all of the neutral species is that the Ce centers in both the molecules and clusters are in the +4 oxidation state. In bulk ceria (CeO$_2$), an unoccupied, narrow 4f band lies between the conventional valence (predominantly O 2p) and conduction (Ce 5d) bands. Within the CeO$_2$−, CeO$_3$H$_2$−, and Ce(OH)$_4$− series, the PE spectra and computational results suggest that the Ce 6s-based molecular orbital is the singly occupied HOMO in CeO$_2$− but becomes destabilized as the Ce 4f-local orbital becomes stabilized with increasing coordination. CeO$_3$−, a hyperoxide, undergoes photodissociation with 3.49 eV photon energy to form the stoichiometric neutral CeO2 and O−. In the CeO$_2$−, Ce$_2$O$_4$− ,and Ce$_3$O$_6$− stoichiometric cluster series, the 6s destabilization with 4f stabilization is associated with increasing cluster size, suggesting that a bulk-like band structure may be realized with fairly small cluster sizes. The destabilization of the 6s-based molecular orbitals can be rationalized by their diffuse size relative to Ce—O bond lengths in a crystal structure, suggesting that 6s bands in the bulk may be relegated to the surface.