BOUND-STATE RESONANCES AND INTERACTION POTENTIAL OF HELIUM SCATTERED BY GRAPHITE (0001)

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 V0 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 V10 is making a relevant contribution to the periodic part of the potential and the matrix elements m|V10|n are evaluated. The energy levels and the matrix elements are used to derive information on the gas-surface potential. V0 is found to be well represented by a Lennard-Jones 5-10 potential, with a well of 15.70 meV. V10 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. © 1979 The American Physical Society.