LEED investigation of the alloying/de-alloying transition in the Cu{100}/Bi system

The surface structures formed by deposition of 0.25 ML and 0.50 ML Bi on Cu{100} at room temperature have been determined quantitatively using the symmetrised automated tensor low energy electron diffraction (SATLEED). At theta(Bi) = 0.25 ML, Bi forms a semi-ordered p(2 x 2) surface alloy with the Bi atoms located at a height of 0.56 +/- 0.06 Angstrom with respect to centre of gravity of the buckled outermost Cu layer ( buckling amplitude = 0.11 Angstrom). The interlayer spacings in the first three substrate layers are found to be: d(12) = 1.71 +/- 0.06 Angstrom, d(23) = 1.82 +/- 0.06 Angstrom, d(34) = 1.81 +/- 0.06 Angstrom (d(bulk) = 1.807 Angstrom). The first and the third Cu layers are found to be buckled by 0.11 +/- 0.06 Angstrom and 0.05 +/- 0.06 Angstrom, respectively. At higher Bi coverage, de-alloying occurs culminating in formation of a well ordered c(2 x 2) overlayer at theta(Bi) = 0.50 ML. Bi atoms occupy the four-fold hollow sites with a vertical Bi-Cu interlayer separation of d(Bi-Cu) = 2.17 +/- 0.06 Angstrom above a slightly perturbed substrate. The interlayer spacing in the first four substrate layers are found to be: d(12) = 1.82 +/- 0.03 Angstrom, d(23) = 1.80 +/- 0.03 Angstrom and d(34) = 1.84 +/- 0.03 Angstrom. A small buckling of 0.02 +/- 0.02 Angstrom is detected in the second Cu layer in which Cu atoms below Bi atoms are rippled outwards. The structures obtained by LEED are compared to those evaluated recently for the same system by surface X-ray diffraction.