H-2 adsorption/desorption at Si(111)-(7x7): a density functional study

The interaction of H-2 with the Si(111)-(7 x 7) surface is investigated by means of density functional slab calculations on a (4 x 2) reconstructed model surface. A viable mechanism for beta(1) desorption is identified, which involves thermally activated SiH2 units at adatom sites. This mechanism leads to adsorption and desorption barriers of 1.0 and 2.4eV, respectively, in agreement with experiment. An explanation for the two components observed in the beta(1) peak of temperature-programmed desorption spectra is proposed. The lattice deformation energy at the transition state for desorption is large (similar to 0.6 eV). If we assume that this remains in the surface after H-2 desorption, the low translational energy of desorbing molecules which has been observed experimentally can in part be explained.