A classical picture of threshold resonances in diffractive systems is
provided in the light of a new classical singularity recently predicte
d for atom surface scattering, the skipping singularity. As an illustr
ation of these dynamics, application to the scattering of He atoms by
the stepped Cu(115) surface is presented using both a hard corrugated
one-dimensional wall and a corrugated Morse potential. It is also emph
asized that these resonances are observable for highly corrugated surf
aces or for surfaces with weak corrugation but at incident conditions
where multiple scattering is important. (C) 1997 Elsevier Science B.V.