Vibrational interactions at surfaces: H2O on Si(100)

The structures and vibrational frequencies of water dissociatively adsorbed on a Si(100)-(2 X 1) surface are investigated by accurate quantum chemical cluster calculations. This Letter addresses the experimentally observed puzzling isotopic shift from the Si-OH stretch (825 cm(-1)) to the Si-OD stretch (840 cm(-1)) measured upon D2O exposure, which is opposite to the decrease in frequency expected from simple mass arguments. Our calculations show that vibrational interactions between the Si-OH stretch and the Si-O-H bending modes account for all the observed effects.