Photoelectron spectrum of PrO^-

The photoelectron (PE) spectrum of PrO^- exhibits a short 835 ± 20 cm^-1 vibrational progression of doublets (210 ± 30 cm^-1 splitting) assigned to transitions from the 4f² [³H₄] σ_6s² Ω = 4 anion ground state to the 4f² [³H₄] σ_6s Ω = 3.5 and 4.5 neutral states. This assignment is analogous to that of the recently reported PE spectrum of CeO^-, though the 82 cm^-1 splitting between the 4f [²F_2.5] σ_6s Ω = 2 and Ω = 3 CeO neutral states could not be resolved [Ray et al., J. Chem. Phys. 142, 064305 (2015)]. The origin of the transition to the Ω = 3.5 neutral ground state is 0.96 ± 0.01 eV, which is the adiabatic electron affinity of PrO. Density functional theory calculations on the anion and neutral molecules support the assignment. The appearance of multiple, irregularly spaced and low-intensity features observed ca. 1 eV above the ground state cannot be reconciled with low-lying electronic states of PrO that are accessible via one-electron detachment. However, neutral states correlated with the 4f² [³H₄] 5d superconfiguration are predicted to be approximately 1 eV above the 4f² [³H₄] σ_6s Ω = 3.5 neutral ground state, leading to the assignment of these features to shake-up transitions to the excited neutral states. Based on tentative hot band transition assignments, the term energy of the previously unobserved 4f² [³H₄] σ_6s Ω = 2.5 neutral state is determined to be 1840 ± 110 cm^-1.