Evidence for a deep bound-state level in the He-4-NaCl(001) potential

Through a systematic high-resolution study of the elastic bound state resonances observed in the inelastic background of the angular distributions of He-4 scattering from the NaCl(001) surface, carried out at 90 K, we obtained clear and conclusive evidence that a deep level in the He-4-NaCl(001) interaction potential exists at -7.07 meV. Important deviations from the free-atom dispersion are observed, as expected for deep bound-state levels. We discuss the physical reasons why deep-level resonances, which are observed in inelastic scattering, may not be detectable in elastic scattering experiments. This finding appears to be crucial with respect to the theory of atom-surface potentials since former quantum-mechanical and close-coupling calculations excluded the existence of a deep level in He-4-NaCl(001) in agreement with elastic scattering studies but at odds with phenomenological predictions. (C) 1997 Elsevier Science B.V.