Observation of zero-field splitting in the anion photoelectron spectra of RhO−: A combined experimental and theoretical study

The amalgamation of high-resolution anion photoelectron spectroscopy and quantum chemistry calculations has been proven to be a powerful approach in probing the fine structures of valence-shell electrons. However, to accurately resolve the fingerprints of the spectra for compounds containing transition-metal remains a formidable challenge because of the high density of low-lying states caused by the partially filled d- and s-orbitals. In our experiments, two adjacent peaks with a separation of 190 ± 16 cm-1 of X band were observed in the photoelectron velocity map imaging of gas-phase RhO-. The theoretical calculations, including spin-orbit effects, predicted the zero-field splitting of the spatially nondegenerate ground states of the anion (3Σ-) and its corresponding neutral molecule (4Σ-) to be 133 and 32 cm-1, respectively. Based on our theoretical calculations, we suggest that the zero-field split states (Σ41/2- and 4Σ3/2-) of RhO- coexist in the gas phase under our experimental condition, and the experimentally observed separation of the X band primarily stems from the detachment of the electron from the two spin-orbit split states of the anion to those of the neutral species. The experimental spectra were further used to determine the electron affinity of RhO (1.581 ± 0.002 eV). Accompanying harmonic vibrational frequencies ωe and anharmonic constants ωeχe of the two species are also obtained.