Surface structure of protonated R-sapphire (1102) studied by sum-frequency vibrational spectroscopy

Sum-frequency vibrational spectroscopy was used to study the protonated R-plane (1102) sapphire surface. The OH stretch vibrational spectra show that the surface is terminated with three hydroxyl moieties, two from AlOH(2) and one from Al(2)OH functional groups. The observed polarization dependence allows determination of the orientations of the three OH species. The results suggest that the protonated sapphire (1102) surface differs from an ideal stoichiometric termination in a manner consistent with previous X-ray surface diffraction (crystal truncation rod) studies. However, in order to best explain the observed hydrogen-bonding arrangement, surface oxygen spacing determined from the X-ray diffraction study requires modification.