Theory of Adsorption and Desorption ofH2/Si(001)

Abstract

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 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 (\lesssim 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.Comment: RevTex file, 11 pages, to appear in Phys. Rev. Lett., Vol. 74 (1995). Figures are not included; to receive a copy of the figures please send your address and FAX number to pehlke@FHI-Berlin.MPG.D