  Experimental and theoretical results for the dissociative adsorption of H_2
on vicinal Si(001) surfaces are presented. Using optical second-harmonic
generation, sticking probabilities at the step sites are found to exceed those
on the terraces by up to six orders of magnitude. Density functional theory
calculations indicate the presence of direct adsorption pathways for
monohydride formation but with a dramatically lowered barrier for step
adsorption due to an efficient rehybridization of dangling orbitals.
