surface: implications for the H2 desorption mechanism

The adsorption of atomic hydrogen on the reconstructed Si ( lOO)-2 x I surface is studied using embedded Si clusters as models of an extended Si surface. Analytic gradients of generalized valence bond (GVB) wavefunctions are used to predict equilibrium structures and harmonic vibrational frequencies; the correlation-consistent configuration interaction (CCCI) method is used to calculate heats ofadsorption. We predict that the first Si-H bond strength of a silicon dimerD,(Si-Si-H) is 86.1 kcal,‘mol, while the second Si-H bond strength D,(HSiSi-H) is 87.9 kcal/mol. Thus, no significant thermodynamic preference exists for either Si-Si-H or H-Si-B-H surface configurations, consistent with recent infrared and scanning tunneling microscopy experiments. The predicted adsorption energetics have important consequences for Hz desorption (A&,=70.7 kcal/mol), with a new mechamsm proposed involving H atom diffusion followed by prepairing desorption of two H atoms on adjacent silicon dimers in the same dimer row.