Bound-state resonances and interaction potential of helium scattered by graphite (0001)

Abstract

The present study was undertaken on the basis of two motivations. On one hand, the He-graphite system is ideally suited for the study of bound-state resonances and of band-structure effects in atom-surface scattering; on the other, an accurate determination of the interaction of a He atom with the basal plane of graphite is of great value for the study of physical adsorption and the properties of two-dimensional adsorbed layers. Elastic diffraction measurements of quasimonochromatic he atoms from a low-temperature graphite (0001) surface are reported here. From the angular position of bound-state resonance minima in the specular intensity, the laterally averaged potential $V_0$ is found to give rise to five discrete levels with energies of 11.98, 6.33, 2.85, 0.99, and 0.17 meV. Band-structure effects taking place at the crossing of resonances were studied for a variety of experimental conditions. From the observed splitting, it is confirmed that only the first Fourier component $V_{10}$ is making a relevant contribution to the periodic part of the potential and the matrix elements $〈{\chi }_m\left|V_{10}\right|{\chi }_n〉$ are evaluated. The energy levels and the matrix elements are used to derive information on the gas-surface potential. $V_0$ is found to be well represented by a Lennard-Jones 5-10 potential, with a well of 15.70 meV. $V_{10}$ is also represented by a model potential, and the overall implication of the results is discussed in the light of other experimental and theoretical findings. A brief description of the line shapes of the resonances and an evaluation of the linewidths are also given; these are compared with recent theoretical studies.