Structural Information from Atomic Beam Diffraction

Abstract

Since about 1975, several novel surface spectroscopic methods based on the interaction of thermal energy neutral particles [4.1] even with very reactive solids have become possible due to the progress in combining high-resolution molecular beam production systems [4.2] with UHV techniques [4.3]. Because of their low kinetic energies (5–300 meV), beams of (nonreactive) atoms or molecules probe the topmost layer of the surface in an absolutely nondestructive manner. Using light species, especially helium, the scattering is predominantly elastic and, as the de Broglie wavelengths are of the order of several tenths of an Angstrom, diffraction effects dominate on well-ordered surfaces [4.4]. Measurements of the angular locations of the Bragg-diffraction beams allow determination of the size and orientation of the surface unit cells and analyses of diffraction intensities yield the surface corrugations, which very often provide direct pictures of the geometrical arrangement of the surface atoms [4.4]. In the diffraction regime, accurate determination of the particle—surface physisorption potential becomes possible via resonant scattering into bound state channels [4.4,5]. In this contribution a brief but self-contained survey is given of the experimental methods used to obtain atomic beam diffraction data and the theoretical procedures used to analyze them quantitatively to arrive at the pertinent corrugation functions. Selected examples of structural results serve to illustrate the potential and scope of atomic beam diffraction and allow discussion of its position relative to other surface structure methods like scanning tunneling microscopy (STM) [4.6] and low energy electron diffraction (LEED) [4.7].