LEAD ON CU(100) AND CU(510) - DESORPTION AND SURFACE-DIFFUSION NEAR THE BULK MELTING TEMPERATURE OF PB

The desorption and surface diffusion of Pb, in the submonolayer range, on both (100) and (510) Cu surfaces are studied under ultrahigh vacuum conditions, in a goniometric chamber coupled to a 2.5 MV Van de Graaff accelerator. Absolute Pb surface concentrations are determined by Rutherford backscattering (RBS), using 500 keV He-4 beams. The results are presented for two treatment temperatures: 573 and 638 K, i.e. slightly below and above the bulk melting temperature of Pb (600 K). The coefficients for diffusion parallel to the [100] step edges of the (510) surface are, on average, 2.5 times higher than on the (100) surface, indicating preferential occupation of step sites by Pb atoms. In the whole range studied (between 0.1 and 1 Pb monolayer), the desorption rate and the diffusion coefficients increase markedly with the concentration. Also, sharp variations of the diffusion coefficient on Cu(510) are observed for Pb coverages around 0.3 monolayer and may be related to faceting, which is known to take place at this Pb concentration.