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 recursion method, we compute the interaction energies for pairs, trimers and tetramers of hydrogen impurities. Surface tetrahedral (A(+)) and subsurface octahedral(A(-)) interstitial positions are considered. We find that the interactions between pairs of A(+) and A(-) impurities located on two first neighbours type-A vertices are more attractive than the lateral interactions. Considering then trimer and tetramer interactions, we point out a tendency to the formation of a(root 3 X root 3)R30 degrees superstructure, with hydrogen in both A(+) and A(-) positions, in agreement with the experimental data.