Theoretical studies of underlayer chemisorption. I. Electronic structure of Ti6and Ti6N clusters

Abstract

The discrete variational (DV)-X alpha cluster method is applied to elucidate the elementary process of chemisorption of the underlayer type forming a nitride layer at the Ti(0001) metal surface. The electronic structures of a Ti metal cluster (Ti6) and the chemisorbed clusters (Ti6N) are calculated. It is found that a characteristic peak of the density of states consisting of odd-parity symmetry orbitals appears in Ti6 at the Fermi level. The peak may correspond to the surface band at the Fermi surface found in the metal film. A specific energy level of Ti6 1.6 eV below the Fermi level is found to be very sensitive to the chemisorption in agreement with photoemission data. This is confirmed by comparing the electronic structure of the Ti6 system with that of the chemisorbed one in which a nitrogen atom sits at the body-centre site of the Ti6 cluster. It is pointed out that the elevation of the -1.6 eV level far above the Fermi level in the chemisorption and the subsequent filling electrons in the surface states of Ti6 would induce a relaxation of the Ti6 framework helping the penetration of a N atom through the first layer. The change of the electronic structure of the Ti6N system when the nitrogen atom moves from the outside to the inside of the Ti6 cluster is also discussed in connection with the deformation process of the Ti metal surface.