Electron Scattering by Diatomic Molecules Adsorbed on Surfaces

Abstract

The types of interaction between a molecule and a surface are conventionally divided into two categories, physisorption and chemisorption according to the strength of bonding between the molecule and the substrate. In this article we confine our discussion to physisorption where the bonding is relatively weak with the result that the properties of the molecule, such as electronic structure and vibrational frequencies, are only mildly perturbed. Of particular interest is the effect of a surface on resonance states due to the importance of these states in a number of electron spectroscopies as well as their role in a wide range of dynamical processes at surfaces, such as photodesorption, molecule-surface scattering and dissociative molecular adsorption. Numerous experiments have been carried out to investigate these effects using high-resolution electron-energy loss spectroscopy (HREELS)1 and low-energy electron diffraction (LEED)2 techniques. Similarly the theoretical investigation of scattering by adsorbed molecules has also intensified. Earlier calculations3,4,5} approximated the effect of a metallic surface using a classical image potential or a modified potential which took account of both image effects and surface reflectivity. However, one of the drawbacks in many of these models was the inadequate target representation. The current approach represents an improvement on this earlier work in that we include a proper target description in an ab initio R-matrix calculation. In addition we have developed a formalism which enables us to exclude the incident wavefunction and calculate the scattering part of the wavefunction directly6. Consequently we are able to circumvent the convergence problems which arise with methods which require the calculation of the total wavefunction.