Studies of the surface deuterioxyl group and adsorbed D₂O on γ-Al₂O₃ using picosecond infrared pump probe spectroscopy

Picosecond infrared-infrared pump-probe experiments in the OD stretching region were carried out on dehydrated and multilayer water-adsorbed γ-Al₂O₃. For the dehydrated γ-Al₂O₃, transient bands assigned to the bleaching and hot bands of the OD stretching mode of isolated surface hydroxyl groups were observed, and the population lifetime (T₁ of the vibrational excited state (v = 1) of the mode was 200 ± 10 ps at 293 K. The characteristic temperature dependence of T₁ is indicative of a seven-phonon process. For the water-adsorbed γ-Al₂O₃, the transient bleaching and transient hot bands with a lifetime of 10-12 ps were observed by excitation of the absorption peak at 2630 cm^-1. In addition to these bands, weak transient bands were observed in the low-frequency region. These results are considered evidence of the existence of isolated water molecules on the γ-Al₂O₃ surface. Pure dephasing is also discussed based on the bandwidth of the observed transient bands.