8 - Atomic and molecular beam scattering

Summary

Introduction

Although many techniques have been developed to study surface properties, most of these techniques are not fully surface-specific and yield information about the surface properties, entangled with information on the first few atom layers. An interaction which can be surface-specific is that between a gas atom or molecule and a surface. This interaction spans a range of phenomena, from diffraction through inelastic scattering to irreversible chemisorption, depending on the nature of the gas—surface potential. Gas atoms or molecules of low kinetic energy (<0.1 eV) act as very soft probes of the surface and, since they are physically unable to penetrate the solid, exhibit an extreme sensitivity to the outermost atomic layer, a sensitivity which surpasses that of LEED or AES. Indeed, one of the attendant difficulties in the development of atomic and molecular beam scattering from surfaces has been the problem of obtaining surfaces which are sufficiently clean to show, for example, any diffraction features which may be present. Surfaces which on examination by AES show no impurities and give sharp, well-defined LEED patterns may, nevertheless, be insufficiently clean for atomic or molecular beam studies. A good example of this sensitivity is provided by the work of Lapujoulade, Lejay & Papanicolaou (1979).