Interaction of H-2 with Si(001)-(2 x 1): Solution of the barrier puzzle

The sticking probability of H-2 on Si(001) is immeasurably small at room te mperature, indicating the presence of a large energy barrier to adsorption. Surprisingly, the final state energy distributions of H-2 molecules desorb ing from Si(001) show no signs of having traversed such a barrier, in appar ent contradiction with microscopic reversibility. Here we report experiment al and theoretical evidence resolving this long-standings puzzle. Adsorptio n and desorption proceeding along two distinct, microscopically reversible pathways can explain all observations.