Temperature programmed desorption of molecular hydrogen from a Si(111) surface: Theory and experiment

Abstract

New experimental temperature programmed desorption (TPD) data have been obtained under carefully controlled conditions for atomic deuterium on single crystal Si(111). A wide range of initial coverages from Θ=1.25 to 0.05 ML was used. It was found that the results could only be satisfactorily interpreted in terms of a two-site adsorption model in which it is suggested that two formally second-order reactions involving the monohydride (deuteride) contribute to the well-known β1 desorption peak at ≊810 K with the relative importance of these two reactions changing with initial coverage. The pre-exponential factors for these reactions were found to be 2 cm2 s−1 and 1 cm2 s−1 with corresponding activation energies of 57.5±2 kcal mol−1 and 56.5±2 kcal mol−1, respectively, for deuterium desorption when the energy difference between the two sites was taken to be 2.5 kcal mol−1. The other desorption channel (β2) was also found to exhibit second-order kinetic behavior involving the dihydride (deuteride). In this case the pre-exponential factor was determined as 0.5 cm2 s−1 and the activation energy as 46.2±2 kcal mol−1. Some suggestions are made concerning the reaction mechanisms and transition states for these desorption processes.