Exotic electronic structures of Sm$_x$Ce$_{3-x}$O$_y$ (x = 0-3; y = 2-4) clusters, and the effect of high neutral density of low-lying states on photodetachment transition intensities

Abstract

Lanthanide (Ln) oxide clusters have complex electronic structures arising from the partially occupied Ln 4$f$ subshell. New anion photoelectron (PE) spectra of Sm$_x$Ce$_{3-x}$O$_y$− (x = 0–3; y = 2–4) along with supporting results of density functional theory (DFT) calculations suggest interesting $x$ and $y$-dependent Sm 4$f$ subshell occupancy with implications for Sm-doped ionic conductivity of ceria, as well as the overall electronic structure of the heterometallic oxides. Specifically, the Sm centers in the heterometallic species have higher 4$f$ subshell occupancy than the homonuclear Sm$_3$O$_y$−/Sm$_3$O$_y$ clusters. The higher 4f subshell occupancy both weakens Sm—O bonds and destabilizes the 4$f$ subshell relative to the predominantly O 2$p$ bonding orbitals in the clusters. Parallels between the electronic structures of these small cluster systems with bulk oxides are explored. In addition, unusual changes in the excited state transition intensities, similar to those observed previously in the PE spectra of Sm$_2$O− and Sm$_2$O$_2$− [J. O. Kafader et al., J. Chem. Phys. 146, 194310 (2017)], are also observed in the relative intensities of electronic transitions to excited neutral state bands in the PE spectra of Sm$_x$Ce$_{3-x}$O$_y$− (x = 1–3; y = 2, 4). The new spectra suggest that the effect is enhanced with lower oxidation states and with an increasing number of Sm atoms, implying that the prevalence of electrons in the diffuse Sm 6$s$-based molecular orbitals and a more populated 4$f$ subshell both contribute to this phenomenon. Finally, this work identifies challenges associated with affordable DFT calculations in treating the complex electronic structures exhibited by these systems, including the need for a more explicit treatment of strong coupling between the neutral and PE.