The kinetics of formation and structure of an underlayer alloy:: the Cu(100)-c(2 x 2)-Pd system

The kinetics and mechanism of an irreversible overlayer to underlayer transition in the Cu(1 0 0)-c(2 x 2) surface alloy has been investigated by LEED. The activation energy for Pd site switching from the outermost layer to subsurface (second layer) sites has been estimated to be 109 +/- 12 kJ mol(-1) (1.13 +/- 0.12 eV). The structure of the underlayer alloy has been examined quantitatively by tensor LEED. The Cu(I 0 0)-c(2 x 2) underlayer is demonstrated to have its origin in substitution of approximately 0.5 ML of Pd into the second layer to form a c(2 x 2) CuPd underlayer alloy capped by a copper monolayer (R-P = 0.28). A mixed third layer c(2 x 2) CuPd layer capped by a copper double layer may be ruled out. Incorporation of sub-surface Pd into the Cu(1 0 0) surface leads to significant expansion of both the first and second interlayer spacings of Delta dz(12) = +3.3% (0.06 Angstrom) and Delta Dz(23) = +6.6% (0. 12 Angstrom) relative to the bulk Cu(1 0 0) interlayer spacing (1.805 Angstrom) leading to a net expansion of the outermost three layer slab of 0.18 Angstrom. (C) 2001 Elsevier Science B.V. All rights reserved.