Dissociative adsorption of H2 on Si(100) induced by atomic H

We report the observation of H-2 adsorption on the H/Si(100) surface using scanning tunneling microscopy. Predosing the surface by atomic H leads to the efficient adsorption of 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, H-2 sticking is strongly inhibited on the clean Si(100)-(2 X 1) surface where the dangling bonds are rendered less reactive by their mutual interaction.