# Regensburg 2004 – wissenschaftliches Programm

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# O: Oberflächenphysik

## O 18: Adsorption an Oberflächen I

### O 18.4: Vortrag

### Dienstag, 9. März 2004, 12:00–12:15, H38

**Angular dependence of H**_{2}** 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 H_{2} 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^{7.8}θ) whereas for *E*_{kin} <
*E*_{a}, H_{2}-momentum parallel to the step edges was found to
enhance dissociation, giving rise, e.g., to a broad polar distribution
(∼ cos^{1.2}θ) for *E*_{kin} = 30 *meV*. This behaviour
contrasts the one previously observed for H_{2} 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)