Hydrogen adsorption and desorption on silicon revisited

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Abstract
Recent molecular beam data on the energy dependence of sticking coefficients for dissociative adsorption of hydrogen molecules on silicon are analyzed in terms of the five dimensional (5D) quantum reactiondynamics used before to describe data on sticking as a function of surface temperature and on state resolved desorption. The sticking coefficient of the 5D model depended strongly on the energy of surface phonons but weakly on the kinetic energy of the incoming H 2 molecules. In a recent molecular beam experiment the latter dependence was measured directly and found to be substantially stronger than concluded from the earlier data. These results have important consequences for the energy and angular distribution of desorbing particles: If the parameters of the 5D model are adjusted so as to describe the new data, it predicts a mean normal kinetic energy of desorbing particles which is now about twice the thermal value. This prediction is rather model independent and can be derived almost directly from the measured data by invoking “detailed balance.” This new increased normal mean energy then is closer to ab initio generalized gradient approximation (GGA) slab calculations predicting about three times the thermal value. The increased kinetic energy also leads to a more forward-peaked angular distribution ∝( cos θ) 11 to 12 instead of ∝( cos θ) 3 to 4 .