Exotic electronic structures of SmxCe3−xOy (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 4f subshell. New anion photoelectron (PE) spectra of SmCeO (x = 0-3; y = 2-4) along with supporting results of density functional theory (DFT) calculations suggest interesting x and y-dependent Sm 4f 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 4f subshell occupancy than the homonuclear SmO/SmO clusters. The higher 4f subshell occupancy both weakens Sm-O bonds and destabilizes the 4f subshell relative to the predominantly O 2p 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 SmO and SmO [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 SmCeO (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 6s-based molecular orbitals and a more populated 4f 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.

Photoelectron Spectra of Gd2O2− and Non-Monotonic Photon-Energy Dependent Variations in Populations of Close-Lying Neutral States

“…Such anti-threshold law behavior was a hallmark of strong electron-neutral interactions observed in previous studies. 10,11,19 The position of manifold II relative to manifold I is nearly identical to a group of excited state transitions observed in the PE spectrum of GdO − , which had been assigned to final neutral states with 4f 7 6p occupancies. 32 Our own calculations on GdO excited states (Table 1) suggest that the final state has 4f 7 5d occupancy, but either way, the transitions are better described as shake-up transitions, where electron promotion accompanies electron detachment.…”

“…We recently reported unusual phenomena observed in the anion photoelectron (PE) spectra of SmxOy suboxide (y  x) cluster anions, along with mixed Ce-Sm suboxides. Specifically, we observed a striking increase in the relative intensities of excited states with decreasing photon energy (decreasing electron kinetic energy, e − KE), [9][10][11] opposite of the Wigner threshold law. 12 This effect was most pronounced in clusters with average oxidation states of the metal centers in the clusters being less than or equal to +2, and it was not at all observed in pure cerium oxide clusters of any stoichiometry.…”

“…We note here that lower-intensity signal at similar e − BE values in numerous LnxOy − (Ln = Ce, Pr, Sm, Eu) suboxide clusters has been observed, and attributed to shake-up transitions. [9][10][11]21,[23][24][25] Included in Figure 3 are the relative energies of excited states that differ by more than one electron from the 16 B1u anion (beyond spin of the electrons in the outer valence orbitals). For example, considering the Why are shake-up transitions prevalent and commonly observed in the spectra of lanthanide oxides with multiply occupied 6s-based orbitals?…”

On the Linear Geometry of Lanthanide Hydroxide (Ln—OH, Ln=La-Lu)

“…26 Similar studies performed on AnC2 and AnCl3 have also implicated actinide 5f orbitals in bond formation. [26][27][28][29][30][31][32] Motivated by our lab's recent interest in metal oxide clusters, [33][34][35][36][37][38] we were intrigued by an apparent dearth of literature exploring the structure and bonding of lanthanide hydroxides. With the goal of exploring the full series, we note the inclusion of PmOH; while included here for completeness, Pm is unstable and thus PmOH is not likely to be experimentally studied.…”

A Density Functional Theory Investigation of the Reaction of Water with Ce2O-

“…In addition, various experimental and theoretical studies have explored the reactivity of classes of cerium-based clusters with various gas-phase small molecules. 17,19,39,42,[44][45][46][47][48][49][50][51][52][53][54][55][56] Of particular interest here are computational studies conducted by Zhou and co-workers on the reactivity of Ce n (n=1-3) with up to six water molecules. 44,45 Their results indicate high reactivity of these cerium clusters and their efficiency in decomposing multiple water molecules.…”