INTERACTIONS BETWEEN HYDROGEN-ATOMS NEAR A PD(111) SURFACE

We present a theoretical investigation of H/Pd(111) for low coverage. In a tight binding scheme, using the generalized phase shift and recur sion method, we compute the interaction energies for pairs, trimers an d tetramers of hydrogen impurities. Surface tetrahedral (A(+)) and sub surface octahedral(A(-)) interstitial positions are considered. We fin d that the interactions between pairs of A(+) and A(-) impurities loca ted on two first neighbours type-A vertices are more attractive than t he lateral interactions. Considering then trimer and tetramer interact ions, we point out a tendency to the formation of a(root 3 X root 3)R3 0 degrees superstructure, with hydrogen in both A(+) and A(-) position s, in agreement with the experimental data.