Symmetry breaking in core-valence double photoionization of SO2

Abstract

Core-valence double photoionization electron spectra of the SO${}_{2}$ molecule involving the S 2$p$ and O 1$s$ inner shells have been measured using a time-of-flight multiparticle coincidence technique. The experimental spectra are compared with quantum-chemical calculations based on density functional theory by which several core-valence dicationic states are identified. Assignments conform with a picture where the formation of a O 1$s$-valence dicationic state is associated with a physical, ``pseudo-Jahn-Teller,'' symmetry breaking and core-hole localization. It is shown that while density functional theory gives very good transition energies in the symmetry-broken case, it gives a poor representation in the symmetry-restricted case, and an incomplete account of the Hartree-Fock localization energy.