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

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 O1s and K2p core-level PES spectra of KO₂, which confirms the good quality of the KO₂ sample employed in this work. The measured O1s 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 O2p electrons. In the O1s XAS, both the unoccupied O2p antibonding π_g and σ_u states are identified. It is found that the unoccupied states, observed in the O1s 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.