Ab Initio Structure and Energetics for the Molecular and Dissociative Adsorption of NH3 on Si(100)-2 × 1

We predict the structures and detailed energetics for the dissociative adsorption of NH 3 to form NH 2 and H adsorbed on a single Si dimer on the Si(100)-2 × 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 NH 3 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 NH 3, 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.