Low Energy Positron and Positronium Diffraction

Abstract

Although this workshop is primarily concerned with positron scattering (and annihilation) with individual atoms in the gaseous state, it is clear that in order to understand the scattering of positrons and positronium Ps from solid surfaces one must often employ concepts directly related to the problem of scattering in gases. In the case of a monolayer of atoms (or molecules) physisorbed to a surface, the physisorbed layer can be treated accurately as a two dimensional gas of weakly perturbed atoms.^l Elastic scattering, for example, of positrons and Ps from such an adsorbed layer would then indirectly contain information about the scattering from the individual adsorbed atoms. This information is of course complicated by the multiple scattering of the incident particle wavefunction by the adsorbed atoms and the atoms of the solid substrate. However if there is two-dimensional periodicity in the adsorbed layer (usually as a consequence of the crystal symmetry of the substrate), the multiple scattering calculations of the scattering intensity are significantly simplified.^2,3 Because of the weakness of the van der Waals’ image force responsible for the physisorbtion bond, it is difficult to produce an ordered absorbed layer.