METASTABLE (2-S-3) HELIUM ATOM SCATTERING FROM NIO(100) AND CU(100) SURFACES

The scattering of metastable 2(3)S He atoms (He) From cleaved NiO(100 ) as well as from clean and CO-covered Cu(100) surfaces has been studi ed. For these varied surfaces, which were characterized in situ by gro und state Hr atom scattering, only broad He angular distributions wit hout and diffraction peaks were observed. For metastable He atoms scat tered from the clean Cu(100) surface a total survival probability of 1 x 10(-6) was determined. For NiO(100) and the CO-covered Cu(100) surf ace values of about 1 x 10(-5) were obtained. Time-of-flight spectra o f the surviving He atoms revealed a substantial energetic broadening which increases with the substrate temperature. This behaviour indicat es a large well depth for the He-surface interaction potential and is discussed in terms of an enhanced multiphonon excitation and/or trapp ing probability upon the scattering. (C) 1998 Elsevier Science B.V.