Dissociative Adsorption ofH2on Si(100) Induced by Atomic H

We report the observation of ${\mathrm{H}}_2$ adsorption on the $\mathrm{H}/\mathrm{Si}\left(100\right)$ surface using scanning tunneling microscopy. Predosing the surface by atomic H leads to the efficient adsorption of ${\mathrm{H}}_2$ in an interdimer configuration of adjacent singly occupied dimers. This strong and local promotion of dissociative adsorption is explained by the noninteracting character of the relevant dangling bonds. By way of contrast, ${\mathrm{H}}_2$ sticking is strongly inhibited on the clean $\mathrm{Si}\left(100\right)-(2\times 1)$ surface where the dangling bonds are rendered less reactive by their mutual interaction.