Multiconfiguration self-consistent-field treatment of H2 desorption from Si(100)-2×1H

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Abstract
The present work studies the symmetric and asymmetric pathways of the recombinative desorption of molecular hydrogen from the monohydride Si(100)‐2×1H surface by the cluster method. The transition states are searched at the self‐consistent‐field and multiconfiguration self‐consistent‐field levels of theory and the energies are calculated at the configuration interaction level of theory. For the asymmetric transition state, the structure determined at the self‐consistent‐field level is similar to that determined at the multiconfiguration self‐consistent‐field level. However, for the symmetric transition state, electron correlation is found to play such a crucial role that multiconfiguration self‐consistent‐field theory has to be used to obtain the saddle point. At the configuration interaction level of theory, the activation energies for the symmetric and asymmetric pathways are computed to be 86.3 and 85.0 kcal/mol, respectively. Compared with the experimental values of 45‐66 kcal/mol, the large barriers suggest that the direct desorption mechanism is not applicable. A multistep desorption mechanism which involves a delocalized process in the formation of dihydride SiH2 and a localized desorption of H2 has been proposed to explain the experimental observations.