DEFECT-MEDIATED ELASTIC RESONANCES IN HE-ATOM SCATTERING FROM SINGLE-CRYSTAL SURFACES

Experimental evidence for a new atom-surface resonant mechanism involv ing surface defects is presented. After an initial selective adsorptio n trapping into surface bound states of a NaCl(001) surface, He atoms are subsequently scattered from surface defects back into the continuu m. The inverse process also occurs: He atoms scatter from a defect and some of the amplitude enters the bound state eventually followed by s elective desorption from the bound state via diffraction. For the 90 d egrees-scattering geometry used, the resonances show up as sharp peaks in the angular distributions of the quasielastic intensity between th e diffraction peaks and are symmetric about the specular peak. A gener al description of the process is given which shows that it is a univer sal feature of any scattering system which can exhibit selective adsor ption type resonances. [SO163-1829(98)00639-0].