Precursor kinetics of dissociative water adsorption on the Si(001) surface

The kinetics of dissociative water adsorption on Si(001) was studied by photoelectron spectroscopy in the temperature range 137-412 K. The valence-band spectra show the well-known features of OH + H and are equal for all coverages from 0.03 monolayers (ML) to saturation at Theta(sat) = 0.5 ML. The adsorbate intensity was measured continuously during exposure at constant pressure. For low T, the sticking coefficient is constant (about one) until it changes to zero with a sharp break when reaching saturation, as is typical for a kinetics dominated by the existence of a mobile precursor state. With increasing temperature, the break smears out because of competition between the chemisorption reaction and precursor desorption. The kinetics is described as a mixture of dissociative chemisorption (as expected) and formally non-dissociative chemisorption. The latter is understandable if dissociation and chemisorption occur on the two dangling bonds of one reconstruction dimer so that each dimer can formally be considered as one site. From the T-dependence, the binding enthalpy of the precursor is estimated to be about -6kT mol(-1). This small value suggests that the precursor is molecular, physisorbed and does not form hydrogen bonds with the surface. Above about 400 K, the initial sticking coefficient begins to decrease and a more strongly bound precursor determines the kinetics.