AVS 46th International Symposium
    Surface Science Division Thursday Sessions
       Session SS2-ThA

Paper SS2-ThA2
Adsorption of Molecular Hydrogen on the Si(100)-2x1 Surface

Thursday, October 28, 1999, 2:20 pm, Room 607

Session: Adsorption at Surfaces
Presenter: E.J. Buehler, University of North Carolina
Authors: J.J. Boland, University of North Carolina
E.J. Buehler, University of North Carolina
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The mechanism of molecular hydrogen adsorption on the Si(100)-2x1 surface was studied using variable temperature scanning tunneling microscopy. The measured desorption barrier of ~2.5eV and the bond energies of Si--H (3.5eV) and H--H (4.5eV) predict a tiny barrier to adsorption. This agrees with data showing that desorbing molecules have no excess translational energy as they leave the surface. The magnitude of the room temperature sticking coefficient (<10@super -10@), however, shows that there is actually a large barrier to adsorption, suggesting a violation of detailed balance. We show that the origin of this discrepancy lies in the motion of the atoms of the Si dimers. The dimers of the clean Si(100)-2x1 surface are constantly tilting at room temperature. We have prepared a surface that contains clean Si--Si dimers that are frozen in a relatively untilted configuration. The room temperature sticking coefficient of molecular hydrogen at these sites is six orders of magnitude greater than that on a clean Si(100)-2x1 surface. The implications for hydrogen adsorption and detailed balance are discussed.