ATOM SCATTERING STUDIES OF STRUCTURALLY DISORDERED SURFACES

Molecular beam scattering experiments have emerged in recent years as an important tool for investigating structural disorder on surfaces. This paper gives a brief overview of the developing theory of scattering from disordered surfaces, which is essential for the interpretation of such experiments, and presents new results on this topic. A brief discussion is given of the methods developed for calculating atom scattering from disordered. systems (e.g. time-dependent quantum wavepackets; distorted-wave approximation; sudden approximation) and of simulations of disordered surface structures (by Monte Carlo or molecular dynamics). The paper focuses, however, on features of the scattering intensities and their relation to disordered surface structures. The main topics include: (1) The concept of cross section for He scattering by an isolated defect on a surface, and its application to the study of interactions between defects. Comsa and his coworkers developed this into a most powerful tool for studying surface defects and defect interactions. We examine the assumptions involved, and the information contained in the measured cross section on defect shape and geometry, and on He/defect potentials. (2) Angular intensity distribution for He scattering from isolated defects; defect rainbow and Fraunhofer effects. (3) He scattering from periodic, substitutionally disordered surfaces. New results are presented on He scattering from mixed Xe + Kr monolayers on Pt(111), comparing theory with the experiments of Comsa et al. The results indicate that these monolayers are indeed periodic and (almost) perfectly substitutionally disordered for all Xe:Kr ratios. Rainbow intensity features due to the substitutional disorder are calculated and analyzed. (4) He scattering from amorphous mixed monolayers, with application to Xe + Ar mixtures on Pt (111). (5) Atom scattering from liquid surfaces, e.g. structure and dynamics of liquid Hg probed by Ar scattering. The paper concludes by emphasizing some of the outstanding open problems in the field.