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

Abstract

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.