Ab initio structure and energetics for the molecular and dissociative adsorption of NH₃ on Si(100)-2 x 1

We predict the structures and detailed energetics for the dissociative adsorption of NH₃ to form NH₂ and H adsorbed on a single Si dimer on the Si(100)-2 x 1 surface at the MRSDCI (multireference single and double excitation configuration interaction) level of theory. We predict that this dissociation involves two steps: (i) barrierless molecular chemisorption of NH₃ followed by (ii) activated N-H bond cleavage of NH_3(a) to form NH_2(a) + H_(a). While the second step involves a barrier, its relatively small height renders the overall reaction barrierless. The extremely high adsorption exothermicity (∼75 kcal/mol) results in a very high desorption barrier. These results can explain the experimentally determined high sticking probability of NH₃, the observation of NH_3(a) at low temperatures, and the observed stability of NH_2(a) and H_(a) on the Si(100) surface up to ∼600 K. Additionally, our CASSCF level (complete active space self-consistent-field) calculated geometries for the dissociatively adsorbed species agree with structures proposed to explain experimental data.