Vibrational interactions at surfaces: H2O on Si(100)

The structures and vibrational frequencies of water dissociatively adsorbed on a Si(100)-(2 × 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.