Theory of Adsorption and Desorption ofH2/Si(001)

While the small sticking coefficient for molecular hydrogen on the Si(001) surface apparently requires a large energy barrier of adsorption, no such barrier is observed in desorption experiments. We have calculated the potential-energy surface of an ${\mathrm{H}}_2$ molecule in front of a Si(001) surface. If we relax the Si substrate, we find an optimum desorption path with a low (≲ 0.3 eV) adsorption energy barrier. While molecules impinging on the surface will mostly be reflected at the larger barrier of some frozen substrate, molecules adsorbed on the surface can desorb along the low-barrier path.