Soft x-ray absorption and photoemission spectroscopy study of superoxide KO2

Abstract

The electronic structure of superoxide KO2 was investigated by employing soft x-ray absorption spectroscopy (XAS) and core-level photoemission spectroscopy (PES). The clean and single component was observed in both the O 1s and K 2p core-level PES spectra of KO2, which confirms the good quality of the KO2 sample employed in this work. The measured O 1s XAS spectrum was compared with the calculated density of states (DOSs), obtained from the first-principles electronic-structure calculations that include both the spin-orbit (SO) effect and the on-site Coulomb interaction (U) between O 2p electrons. In the O 1s XAS, both the unoccupied O 2p antibonding pi(g) and sigma(u) states are identified. It is found that the unoccupied states, observed in the O 1s XAS, agree reasonably well with the calculated DOSs, which suggests the importance of the SO and U effects for KO2 having the high-symmetry tetragonal structure.