A dynamically and kinetically consistent mechanism forH2adsorption/desorption from Si(100)-2×1

Abstract

Curiously, ${\mathrm{H}}_2$ desorption from Si(100)-2×1 follows approximately first-order kinetics rather than the expected second-order kinetics, arousing interest about the mechanism involved in the desorption process. We investigate the energetics and rate constants of three proposed mechanisms for ${\mathrm{H}}_2$ desorption from Si(100)-2×1, namely, the prepairing mechanism, the isomerization mechanism, and the isolated dihydride mechanism, using complete active space self-consistent-field and multireference single- and double-excitation configuration-interaction calculations. We find the desorption barrier for the isolated dihydride mechanism to be 2.49 eV, the only barrier in excellent agreement with the experimentally determined barrier (∼2.5 eV). The isolated dihydride mechanism also provides the only calculated desorption rate constant close to experimental values. Finally, we show that this mechanism is able to explain the experimentally observed apparent violation of detailed balance of ${\mathrm{H}}_2$ adsorption/desorption on Si(100), as well as other experimentally observed dynamics. © 1996 The American Physical Society.