THE ELECTRONIC-STRUCTURE OF KMNO4 INVESTIGATED BY PHOTOEMISSION AND ELECTRON-ENERGY-LOSS SPECTROSCOPY

From photoemission and electron-energy-loss data the following picture of KMnO4, with Mn(VII) (with a formal charge state Mn7+ (3 d0)) tetrahedrally surrounded by four O2--ions, is deduced: strong covalent bonding between Mn(VII) and O2- leads to a considerable occupation of the Mn-3d shell. The ground state of the (MnO4)-1 molecule is an orbital and spin singlet as seen by the absence of any multiplet splitting in the Mn core levels. The valence band shows a four peak structure extending form 4 eV to 8 eV below the Fermi energy. The first peak at 4.2 eV has mainly O-2p character. The remaining peaks are of strongly mixed Mn-3d/O-2p character due to the covalent bonding. This mixing decreases with increasing binding energy. The electron energy loss data show a variety of structures between 2 eV and 10 eV independent of the primary electron energy which defines them as dipole allowed charge-transfer transitions. An additional excitation at 1.8 eV decreases quickly in intensity with increasing electron energy which classifies it as a dipole or spin forbidden transition in the compound. This energy is close to the value of 1.6 eV reported for the activation energy observed in electrical transport data. The results are compared to quantum chemical molecular orbital calculations of the (MnO4)-1 molecule.