Low energy states of NdO⁺ probed by photoelectron spectroscopy

The ionization energy (IE) of NdO and the low-energy electronic states of NdO⁺ have been examined by means of two-color photoionization spectroscopy. The value obtained for the IE, 5.5083(2) eV, is 0.54 eV higher than previous estimates. This leads to the conclusion that the autoionization reaction Nd + O → NdO⁺ + e^- is exothermic by 1.76(10) eV. Thirty vibronic levels of NdO⁺ arising from eight electronic states were observed with partial rotational resolution. The energy level pattern and supporting electronic structure calculations indicated that all of the observed states correlated with the Nd³⁺(4f³, ⁴I)O^2- configuration. The structure was consistent with a ligand field theory model where the electronic states of the Nd³⁺(4f³, ⁴I) atomic ion define a repeated motif in the electronic state energy intervals of the molecular ion. Comparisons with UO⁺ show close similarity in the electronic structures of these isoelectronic species.