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 cov erage range has been followed by monitoring in a helium scattering exp eriment the intensity, position and full width at half maximum of sele cted diffraction peaks. Up to Pb coverages theta similar to 0.2 the sc attered intensity of the (- 1,0) peak, absent on the bare Cu surface, increases as theta(2), thus indicating the ''decoration'' of the subst rate 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 existe nce of two levels is evidenced, one being the substrate covered by lat tice gas, the other that of islands of the condensed phase. The differ ence 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.