Investigation of Surface Imperfections by Diffuse Scattering of He Atoms

Abstract

Helium atom scattering (HAS) is a well established technique for studying the physical properties of atoms and molecules [5.1] in the gas phase. The decrease in intensity of a He atom beam caused by the target molecules allows the determination of the integral cross-section of a molecule, which is related to its total size. Since in a typical molecular beam experiment the de Broglie wavelength of the He atoms is of the order of the diameter of the target molecule (several Å), interference effects will generally cause a variation of the integral cross-section with the energy of the He atoms. This dependence can be used to draw conclusions about the shape of the intermolecular potential. With a more sophisticated molecular beam machine it is additionally possible to measure the angular distribution of the scattered He atoms. The differential cross-section, which can be evaluated from these data, contains a rich structure, again originating from interference between different scattering paths. A careful analysis of these structures allows one to draw much more detailed conclusions on the form of the potential, e.g. one can determine the contour of zero potential [5.2]. In some special cases high precision data for the differential cross-section has even allowed a potential inversion [5.1].