Adsorption of hydrogen atoms on the Si(100)-2×1 surface: implications for the H₂ desorption mechanism

The adsorption of atomic hydrogen on the reconstructed Si(100)-2×1 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 of adsorption. We predict that the first SiH bond strength of a silicon dimer D₀(SiSiH) is 86.1 kcal/mol, while the second SiH bond strength D₀(HSiSiH) is 87.9 kcal/mol. Thus, no significant thermodynamic preference exists for either SiSiH or HSiSiH surface configurations, consistent with recent infrared and scanning tunneling microscopy experiments. The predicted adsorption energetics have important consequences for H₂ desorption (ΔE_des=70.7 kcal/mol), with a new mechanism proposed involving H atom diffusion followed by pre-pairing desorption of two H atoms on adjacent silicon dimers in the same dimer row.