The kinetics of formation and structure of an underlayer alloy: the Cu(1 0 0)-c(2×2)–Pd system

The kinetics and mechanism of an irreversible overlayer to underlayer transition in the Cu(1 0 0)-c(2×2) surface alloy has been investigated by LEED. The activation energy for Pd site switching from the outermost layer to sub-surface (second layer) sites has been estimated to be 109±12 kJ mol−1 (1.13±0.12 eV). ructure of the underlayer alloy has been examined quantitatively by tensor LEED. The Cu(1 0 0)-c(2×2) underlayer is demonstrated to have its origin in substitution of approximately 0.5 ML of Pd into the second layer to form a c(2×2) CuPd underlayer alloy capped by a copper monolayer (Rp=0.28). A mixed third layer c(2×2) 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 Δdz12=+3.3% (0.06 Å) and ΔDz23=+6.6% (0.12 Å) relative to the bulk Cu(1 0 0) interlayer spacing (1.805 Å) leading to a net expansion of the outermost three layer slab of 0.18 Å.