Stereochemistry on Si(001): Angular Dependence of $H_2$ Dissociation

The angular dependence of the dissociative adsorption of molecular hydrogen at terrace and step sites of vicinal single-domain Si(001) surfaces was investigated by means of molecular beam techniques and optical second-harmonic generation. A strongly anisotropic behavior was observed for terrace adsorption with polar distributions of ${cos}^3\vartheta$ and ${cos}^{12}\vartheta$ parallel and perpendicular to the dimer, respectively. The $D_{\mathrm{B}}$-steps show enhanced reactivity under glancing incidence in the upwards direction. The results are traced back to the directionality of the covalent surface bonds.