REACTION DYNAMICS OF MOLECULAR-HYDROGEN ON SILICON SURFACES

Experimental and theoretical results on the dynamics of dissociative a dsorption and recombinative desorption of hydrogen on silicon are pres ented. Using optical second-harmonic generation, extremely small stick ing probabilities in the range 10(-9)-10(-5) could be measured for H-2 and D-2 on Si(111)7X7 and Si(100)2X1. Strong phonon-assisted sticking was observed for gases at 300 K and surface temperatures between 550 K and 1050 K. The absolute values as well as the temperature variation of the adsorption and desorption rates show surprisingly little isoto pe effect, and they differ only little between the two surfaces. These results indicate that tunneling, 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 lattice vibrations. Theoretically, an effective five-d imensional model is presented taking lattice distortion, corrugation, and molecular vibrations into account within the framework of coupled- channel calculations. While the temperature dependence of the sticking is dominated by lattice distortion, the main effect of corrugation is a reduction of the preexponential factor by about one order of magnit ude per lateral degree of freedom. Molecular vibrations have practical ly no effect on the adsorption/desorption dynamics itself, but lead to vibrational heating in desorption with a strong isotope effect. Ab in itio calculations for the H-2 interaction With the dimers of Si(100)2X 1 show properties of the potential surface in qualitative agreement wi th the model, but its dynamics differs quantitatively from the experim ental results.