SMALL ALKALI-METAL CLUSTERS ON (001)QUARTZ SURFACE - ADSORPTION AND DIFFUSION

The adsorption of small (Na)(n) clusters n = 2-8 and 13 on the clean ( 001) quartz surface is studied on the basis of a classical discrete mo del for the clusters and the surface. Both the cluster and the surface are assumed to keep their geometry before adsorption. The equilibrium sites and the stable orientations of the clusters are determined and the adsorption energy at 0 K is shown to increase with n from 4.40 eV (n = 2) to 7.51 eV (n = 13). The diffusion of the cluster at the surfa ce is generally easier for larger n values. The force constant associa ted with the cluster-surface perpendicular bond does not vary signific antly with n when n greater than or equal to 5 since it remains at aro und 300 N m(-1). As a consequence of the cluster mass increase with n, the adbond frequency decreases monotonically. The asymptotic sphere m odel used to describe the clusters fails in interpreting the adsorptio n characteristics when n less than or equal to 13.