HELIUM ATOM SCATTERING FROM ISOLATED CO MOLECULES ON COPPER(001)

Angular distributions have been measured for helium atoms scattering f rom isolated CO molecules chemisorbed on a Cu(001) surface as a functi on of incident beam energy between 9.4 and 100 meV and surface coverag e from 1.3% ML to 9.3% ML. Up to five oscillations an clearly observed in the angular distributions. The parallel wave vector transfers of t he peaks vary only slightly with incident energy and are independent o f coverage up to 20% of a c(2 x 2) layer. New hard wall scattering cal culations show that all of the distinct peaks observed can be explaine d by interference structures involving both Fraunhofer diffraction and illuminated face scattering from CO molecules with an approximate har d wall radius of 2.4 Angstrom with no evidence of the classical rainbo ws predicted in several recent theoretical studies. (C) 1996 American Institute of Physics.