Influence of Flipping Si Dimers on the Dissociation Pathways of Water Molecules on Si(001)

The dissociative adsorption of water molecules on Si(001) was studied up to 850 K by scanning tunneling microscopy. Water molecules dissociate into OH and H fragments to form on-dimer (OD) and interdimer (ID) configurations, and the population ratio n ID/n OD is measured to change from ∼5 at room temperature to the saturation value of ∼0.6 above 500 K, contradicting to the prediction from the static transition-state theory. The sticking coefficient also shows an abnormal temperature dependence. The temperature dependence in the H dissociation and in the molecular adsorption can be elucidated only by taking into account the flip-flop motion of Si dimers.