Regensburg 2004 – scientific programme

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O: Oberflächenphysik

O 18: Adsorption an Oberflächen I

O 18.4: Talk

Tuesday, March 9, 2004, 12:00–12:15, H38

Angular dependence of H₂ dissociation at D_B steps of Si(001) — •C. Stanciu and U. Höfer — Fachbereich Physik, Philipps-Universität Marburg, D-35032 Marburg

The dynamics of dissociative adsorption of molecular hydrogen at the D_B steps of Si(001) was investigated by performing angular dependent measurements of the initial sticking coefficient for different kinetic energies of H₂ from a supersonic molecular beam. Optical second-harmonic generation (SHG) was used as a sensitive monitor of hydrogen coverage at the steps sites. For E_kin = 350 meV, a kinetic energy that considerably exceeds the mean barrier height of E_a = 80 meV, the polar distribution is strongly forward-peaked (∼ cos^7.8θ) whereas for E_kin < E_a, H₂-momentum parallel to the step edges was found to enhance dissociation, giving rise, e.g., to a broad polar distribution (∼ cos^1.2θ) for E_kin = 30 meV. This behaviour contrasts the one previously observed for H₂ adsorption on the terraces [1]. A sharp angular distribution, directed along the direction of the dangling bonds, was observed in that case for E_kin < E_a, indicating a strong lateral corrugation of the barrier height due to the high directionality of the dangling bonds. The behaviour at the steps, however, suggests the dominant influence of a geometric corrugation. Possible origins of such a lateral variation of the barrier position parallel to the step edges, and effects the steering of low energetic molecules will be discussed.

[1] M. Dürr and U. Höfer, Phys. Rev. Lett. 88, 076107 (2002)