Interaction between hydrogen atoms near a Pd(001) surface: the electronic structure

Abstract

We study the electronic structure of N interstitial impurities near a metallic surface in the tight binding model. The use of the generalized phase-shift provides an efficient way to obtain self-consistent expressions for the local densities of states and the interaction energies between impurities. We study the stability of the system of interacting impurities, as a function of their relative position in a semi-infinite host. This approach is not restricted to the most stable positions of the impurities, but is also valid for metastable diffusive configurations. This formalism is applied to the case of pairs and trios of hydrogen impurities near a Pd(001) surface. We consider two types of positions for the hydrogen atoms: octahedral and tetrahedral sites. The pair and trio interactions are found to be very sensitive to the position of the impurities to the surface. Very good agreement with experimental results is found. In the bulk, the “criterion of Switendick” (no interaction if the distance H-H is smaller than 2.1 Å) is satisfied. The preferential adsorption position is the four-fold “hollow” position. The values of the pair and trio interaction energies for adsorbates lead to the c(2 × 2) observed superstructure. No “subsurface” chemisorption is found. The interaction between the hydrogen atoms is mainly “indirect”, mediated through the substrate. Finally this work shows that the trio interaction energies can be written, only roughly, as the sum of relevant pair interaction energies.