Reactions of hydrogen on silicon: Kinetics and dynamics

Hydrogen molecules desorbing from silicon exhibit very low translational heating in spite of a high adsorption barrier. This "barrier puzzle" can be explained in terms of a strong phonon coupling leading to a strong energy transfer to phonons in desorption and, vice versa, to a strong phonon-assisted sticking. Recent kinetic and scanning tunneling microscopy data at higher coverages indicate the existence of a new barrierless reaction path which is supposed to dominate the high coverage reaction dynamics. However, recent data on translational and earlier results on vibrational heating at higher coverage indicate the presence of a high adsorption barrier well in accordance with low-coverage data on vibrationally and translationally assisted sticking. For the solution of this barrier puzzle we present kinetic and dynamic model calculations. We use nonequilibrium thermodynamics to formulate the coupled adsorption-desorption kinetics allowing for different weights for the 2H and 4H processes. We perform four-dimensional coupled channel calculations to treat the dependence of phonons, molecular vibrations and corrugation on these two reaction mechanisms. These calculations indicate that dramatic changes in the vibrational and translational dynamics due to the new barrierless reaction path may occur only near monolayer coverage.