Electronic structures and spin frustration in Ln₃O (Ln = Ce, Sm, Gd) neutrals and anions determined by anion photoelectron spectroscopy

The results of a combined experimental and computational study on Ln₃O (Ln = Ce, Sm, and Gd) anion and neutral clusters are presented and analyzed. These three Ln’s were specifically targeted because they vary in their spin state and orbital angular momentum associated with the 4f^N subshell occupancies. From the anion PE spectra of Ce₃O⁻, Sm₃O⁻, and Gd₃O⁻ measured with 2.330 and 3.495 eV photon energies, we determine the adiabatic electron affinities of the corresponding neutrals to be 0.83 ± 0.03, 1.11 ± 0.05, and 1.17 ± 0.05 eV, respectively. The lowest energy features in all three spectra can readily be reconciled with molecular structures in which the O-atom is central to all three Ln centers, with Ce₃O⁻/Ce₃O assuming pyramidal structures and Sm₃O⁻/Sm₃O and Gd₃O⁻/Gd₃O assuming planar structures. Computationally, the lowest-energy structure of neutral Ce₃O is a kite-like structure, which is not consistent with the observed spectrum. The kite-like and pyramidal structures of Ce₃O⁻ are predicted to be nearly isoenergetic. Electronic states in which all three 4f^N centers are ferromagnetically coupled are predicted to be energetically favored for all species, but spin-frustrated states in which one 4f^N center is antiferromagnetically coupled to the remaining centers are computed to lie 0.05 eV higher in energy than the FM-coupled states for Ce₃O⁻ and Sm₃O⁻. The PE spectrum of Sm₃O⁻ exhibits striking anomalies in the photoelectron angular dependence. This effect is attributed to strong photoelectron–valence electron interactions that drive nominally forbidden changes in the M_f state of the remnant neutral.