HE SCATTERING FROM RANDOM ADSORBATES, DISORDERED COMPACT ISLANDS, ANDFRACTAL SUBMONOLAYERS - INTENSITY MANIFESTATIONS OF SURFACE DISORDER

A theoretical study is made on He scattering from three basic classes of disordered adlayers: (a) translationally random adsorbates, (b) dis ordered compact islands, and (c) fractal submonolayers. The implicatio ns of the results to experimental studies of He scattering from disord ered surfaces are discussed, and a combined experimental-theoretical s tudy is made for Ag submonolayers on Pt(111). Some of the main theoret ical findings are: (1) The scattering intensities from the three disor der classes differ significantly, and can be used to distinguish betwe en them. (2) Structural aspects of the calculated intensities from tra nslationally random clusters were found to be strongly correlated with those of individual clusters. (3) For fractal islands, just as for al l surfaces considered here, the off-specular intensity depends on the parameters of the He/Ag interaction, and does not follow a universal p ower law as previously proposed in the literature. In the experimental -theoretical study of Ag on Pt(111), we use experimental He scattering data from low-coverage (single adsorbate systems to determine an empi rical He/Ag-Pt potential of good quality. Then, we carry out He scatte ring calculations for high coverage and compare with experiments for t hese systems. The conclusion is that the actual experimental phase cor responds to small compact Ag clusters of narrow size distribution, wit h partial translational disorder. (C) 1997 American Institute of Physi cs.