A THEORETICAL-STUDY OF ATOMIC FLUORINE CHEMISORPTION ON THE NI(100) SURFACE

Cluster size convergency of the chemisorption energy has been studied for the case of fluorine on Ni (100). Bond preparation of the cluster is found to be equally important for fluorine as for the previously studied hydrogen chemisorption. An estimate of the chemisorption energy for fluorine in the fourfold hollow site of Ni (100) is reached based on the average value for the bond-prepared clusters, a correction for the use of one-electron ECP's, an estimate of the basis set limit and finally adding the effect of 3d correlation. The chemisorption energy is in this way estimated to be about 120 kcal/mol. Even though the bonding between fluorine and the surface should be regarded as almost totally ionic, there is still no correspondence between the chemisorption energy for a cluster and the highest ionization energy (Fermi level) of that cluster. The critical feature of bond preparation is that it allows the fluorine lone-pair electrons pointing down towards the surface to be fitted into the electronic structure of the cluster.