${\mathrm{H}}_2$ adsorption on vicinal silicon (001) surfaces with double atomic height $D_B$ steps has been investigated using scanning tunneling microscopy. At room temperature, the only observable reaction is dissociative adsorption at step sites. This process leads to H adsorption at pairs of adjacent step atoms aligned with the dimer rows of the upper terrace. The sticking probability is found to increase at sites next to those already occupied by adsorbed H, as manifested in a strongly nonstatistical distribution of occupied pairs of dangling bonds. At elevated surface temperature, dissociative adsorption is also observed to occur at the complementary pairs of step atoms.

• Received 25 October 2000

DOI:https://doi.org/10.1103/PhysRevB.63.121315