Reaction path for hydrogen adsorption and desorption on Si(100)-(2X1)

The intradimer pre-paired desorption mechanism for hydrogen on Si(100)-(2 x 1) has been studied with density functional calculations using 1-dimer and 3-dimer cluster models of the surface. We find that adsorption/desorption occurs in a two-step process through a metastable dihydridelike intermediat e, Two transition states are identified in this pathway. We confirm that th e transition states are at saddle points by computing the vibrational frequ encies, and that the reaction path from the monohydride to the desorbed sta te goes through the intermediate by performing. eigenvector-following calcu lations from the two transition states. The effects of clutter size-and bas is set on the energetics are investigated. It is observed that energetics f rom 3-dimer cluster B3LYP6-311G** calculations are in reasonable agreement with experimental data for both the adsorption and desorption barriers. We find an adsorption barrier of 0.65 eV and a desorption barrier of 2.94 eV: We obtain a distance of approximately 3.21 Angstrom between the silicon dim er atoms in the intermediate structure, implying a broken dimer bond, which we confirm by considering the electron density plots for the structure. Th us our results suggest that during each adsorption or desorption event the dimer bond is broken and reformed. The forces acting on the silicon dimer a toms and the hydrogen atoms along the reaction path are also investigated a nd these provide a picture of the coupling of the adsorption/desorption pro cess to surface vibrations, particularly to the dimer bond breaking and ref orming. This occurrence of dimer bond breaking and reforming along the reac tion _ path suggests an explanation for the experimentally observed large s urface-temperature activation of the adsorption process. (C) 2001 American Institute of Physics.