Photoelectron spectroscopy of cryogenically cooled NiO slow photoelectron velocity-map imaging.

High-resolution anion photoelectron spectra of cryogenically cooled NiO anions, obtained using slow photoelectron velocity-map imaging (cryo-SEVI), are presented in tandem with coupled cluster electronic structure calculations including relativistic effects. The experimental spectra encompass the X̃Σ ← X̃Π, ãΠ ← X̃Π, and ÃΠ ← X̃Π photodetachment transitions of linear ONiO, revealing previously unobserved vibrational structure in all three electronic bands. The high-resolution afforded by cryo-SEVI allows for the extraction of vibrational frequencies for each state, consistent with those previously measured in the ground state and in good agreement with scalar-relativistic coupled-cluster calculations. Previously unobserved vibrational structure is observed in the ãΠ and ÃΠ states and is tentatively assigned. Further, a refined electron affinity of 3.0464(7) eV for NiO is obtained as well as precise term energies for the ã and à states of NiO of 0.3982(7) and 0.7422(10) eV, respectively. Numerous Franck-Condon forbidden transitions involving the doubly degenerate bending mode are observed and ascribed to Herzberg-Teller coupling to an excited electronic state.