The electronic structure of the (100) surface of NiO has been studied
using EELS (electron energy loss spectroscopy) and ab initio calculati
ons. 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 w
ith an adsorbate, i.e. adsorption of NO leads to a shift to higher ene
rgy. 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 ide
ntified by a comparison of the experimental data with theoretical resu
lts.