ADSORPTION-DESORPTION OF H-2 SI - A 5-D DYNAMICAL MODEL/

Several theoretical and experimental results on the dynamics of dissoc iative adsorption and recombination desorption of hydrogen on silicon are reviewed. Theoretically a 5-D model is presented taking lattice di stortion, corrugation and molecular vibrations into account besides th e translational motion of the molecule. The properties of this model a re investigated within the framework of coupled channel calculations. While the temperature dependence of sticking is dominated by lattice d istortion, tile main effect of corrugation is a reduction of the pre-e xponential factor by about one order of magnitude per lateral degree o f freedom. Molecular vibrations have little effect on tile adsorption- desorption dynamics itself but lead to vibrational heating in desorpti on with a strong isotope effect. Ab initio calculations for the H-2 in teraction with the dimers of Si(100) 2 x 1 show properties of the pote ntial surface in qualitative agreement with the model, but its dynamic s differs quantitatively from the experimental results. Experimentally extremely small sticking probabilities in the range 10(-9) to 10(-5) could be measured using optical second-harmonic generation (SHG) for H -2 and D-2 on Si(111)7 x 7 and Si(100) 2 x 1. Strong phonon-assisted s ticking was observed for gases at 300 K and surface temperatures betwe en 550 and 1050 K. The absolute values as well as the temperature vari ation of the adsorption and desorption rates show surprisingly little isotope effect and differ only little between the two surfaces. These results indicate that tunnelling, molecular vibrations, and the struct ural details of the surface play only a minor role for the adsorption dynamics. Instead: they appear to be governed by the localized H-Si bo nding and Si-Si vibrations. These results together with previously mea sured energy and angular distributions of desorbing molecules can be d escribed very well with the theoretical model.