Investigation of the low-lying excited states of PuO22+ Original Research Article

We present a theoretical study of the first low-lying excited states of plutonium, Pu(VI)O22+. Their geometries were optimized and found to be basically unchanged due to the local character of the excitations. Vertical spin-free excitation energies were calculated for all (23) states below 240 kcal/mol. Fine-structure levels were obtained for all states in the f-manifold below 105 kcal/mol. The ground state of the plutonyl(VI) ion is a 3Hg at the spin-free level, and its Ω=4 component at the spin-orbit level. The spin-orbit interaction was calculated at the quasi-degenerate perturbation level, using `the one-to-one correspondence of the integralsʹ method proposed, combined with a shift technique. We propose a simpler approach avoiding the use of generalised contracted ECP basis sets. This is, to our knowledge, the first systematic study of the low-lying excited states of the plutonyl ion including spin-orbit effects.