ELECTRONIC SURFACE-STATES OF NIO (100)

The electronic structure of the (100) surface of NiO has been studied using EELS (electron energy loss spectroscopy) and ab initio calculations. In addition to the previously documented bulk excitations of NiO, two new states at energies of 0.57 and 1.62 eV have been found. These states are attributed to d-d transitions of the nickel surface ions. As expected for surface states, they are affected by the interaction with an adsorbate, i.e. adsorption of NO leads to a shift to higher energy. Ab initio cluster calculations show that the electronic structure of the surface is considerably different from that of the bulk which is a result of the lower symmetry of the crystal field at the surface (O(h) --> C4v). The nature of the observed surface states has been identified by a comparison of the experimental data with theoretical results.