STUDY OF THE SELECTIVE ADSORPTION PHENOMENON IN THE HE-4/CU(11-ALPHA) (WITH ALPHA = 0, 3, 5, 7) ELASTIC-SCATTERING - THE CRITICAL KINEMATIC EFFECT

In a recent paper [Surf. Sci. 251/252 (1991) 369] we have presented a new analysis of the selective adsorption phenomenon within the close-coupling formalism, applied to the He-4/Cu(110) elastic scattering. That analysis is extended here to the series of stepped surfaces He-4/Cu(11-alpha) (alpha = 3, 5, 7). For incident energies from 5 to 90 meV, internal and external widths are obtained as a function of the face index-alpha. Special attention is focussed on the critical kinematic effect, i.e., an enhancement of resonance widths at certain values of the scattering geometry (modulus and angles of the wave vector of the incident atom). At these incident conditions, a remarkable loss of the typical (Lorentzian or Fano) behavior is exhibited by the line shapes of the diffraction intensities. These features are shown in detail for the He-4/Cu(115) system, where the incident conditions are suitable for an experimental verification. A comparison between experimental and theoretical results at 21 meV is also presented.