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

Paper SS2-ThA1
Dissociative Adsorption of H@sub 2@ on Si(001): Reaction Channels and Structure Sensitivity

Thursday, October 28, 1999, 2:00 pm, Room 607

Session: Adsorption at Surfaces
Presenter: M.B. Raschke, Max-Planck-Institut für Quantenoptik, Germany
Authors: M.B. Raschke, Max-Planck-Institut für Quantenoptik, Germany
M. Dürr, Max-Planck-Institut für Quantenoptik, Germany
U. Höfer, Max-Planck-Institut für Quantenoptik, Germany
Correspondent: Click to Email
Making use of the high adsorbate sensitivity of second-harmonic generation (SHG) the dissociative adsorption of H@sub 2@ on Si(001) could be detected in previous experiments.@footnote 1@ The extremely small sticking coefficients were found to exhibit a pronounced dependence on surface temperature, which indicates the importance of dynamic lattice distortions. In the present investigations, by preadsorbing atomic hydrogen, configurations are realized where, e.g., one of the two dangling-bonds per dimer becomes hydrogen terminated. In contrast to the dissociation on the initially clean Si(001) surface this preparation scheme facilitates two additional inter-dimer reaction pathways occuring with either a neighboring identical half-hydrogen-terminated dimer in the cis-configuration or a clean Si-dimer. Studying the dependence of the reactivities with kinetic energy of the H@sub 2@ molecules using molecular beam techniques, in the first case the dissociation was found to be non-activated, whereas a barrier of 0.2 eV was determined for the second. Together with the barrier of 0.8 eV found for dissociation on the clean surface this corresponds to differences in reactivity up to seven orders of magnitude for a surface temperature of 400 K. Comparing the activation with surface temperature with the dependence on H@sub 2@ beam energy a complementarity between exciting surface and molecular degrees of freedom was found. These results are particularly remarkable because of the geometric similarities of the different configurations of the Si-atoms involved. This together with the results of the recent study of the dissociation at D@sub B@-steps on Si(001)@footnote 2@ provide a systematic understanding of the parameter governing the H@sub 2@/Si(001) interaction. @FootnoteText@ @footnote 1@P. Bratu, K.-L. Kompa, and U. Höfer, Chem. Phys. Lett. 251 (1996) 1. @footnote 2@P. Kratzer, E. Pehlke, M. Scheffler, M. B. Raschke, and U. Höfer, Phys. Rev. Lett. 81 (1998) 5596.