A CHANNELING STUDY OF LEAD SUBMONOLAYERS DEPOSITED ONTO CU(100)

We have studied the location of Pb atoms on Cu(100) at submonolayer co verages using a few hundred keV He+ ion beams in channeling experiment s under single- and double-alignment geometry. Different shadowing eff ects of Pb on Cu are experimentally observed according to the structur e of the deposit, which was evidenced by LEED (in particular the c(5 r oot 2 x root 2)R45 degrees and c(2 x 2) structures). These effects dis appear around the melting temperature of the structures. For a Pb cove rage of theta approximate to 0.5, when a c(2 x 2) structure is formed, the distance d(Pb) between the Pb plane and the first (100) Cu plane, measured under double alignment geometry (channeling along [100], blo cking along [110] 45 degrees off the normal to the surface), is d(Pb) = 2.05 + 0.05 Angstrom. We also find that the Pb atoms induce a static disorder in the first Cu surface plane. This disorder may be describe d by isotropic Gaussian displacements of Cu atoms, with a one-dimensio nal projected RMS deviation u(d) of around 0.12 Angstrom. It depends l ittle on the Pb structure: in particular, there is no specific disorde ring in the alloyed state corresponding to low Pb coverages. From this observation, we deduce that for low coverage, the Pb atoms lie about 1 Angstrom above the first Cu plane.