2-DIMENSIONAL OVERGROWTH IN THE LOW SUBMONOLAYER RANGE - THE CASE OF C(2X2)-PB/CU(110)

The vacuum deposition of lead on a copper (110) surface in the low coverage range has been followed by monitoring in a helium scattering experiment the intensity, position and full width at half maximum of selected diffraction peaks. Up to Pb coverages theta similar to 0.2 the scattered intensity of the (- 1,0) peak, absent on the bare Cu surface, increases as theta(2), thus indicating the ''decoration'' of the substrate by a lattice gas of adsorbed Pb atoms. Above this coverage to the completion of the condensed layer with c(2 X 2) structure, the existence of two levels is evidenced, one being the substrate covered by lattice gas, the other that of islands of the condensed phase. The difference in height between these levels favours the assumption that the Pb atoms of the lattice gas replace the Cu atoms of the outermost layer of the substrate. The thermal behavior of the scattered intensities of the two phases gives an indirect indication in the same sense.