Infrared-visible sum-frequency spectroscopy was used to probe surface vibrations of $\alpha$-quartz(0001) under ambient conditions. Two modes at 880 and $980{\mathrm{cm}}^{-1}$ were observed and identified as arising from Si-O-Si and Si-OH structures of the surface. Heat treatment and hydroxylation could convert Si-OH to Si-O-Si and vice versa. The technique is generally applicable to studies of surface phonons of other oxides, semiconductors, and insulators.

• Received 26 March 2008

DOI:https://doi.org/10.1103/PhysRevLett.101.016101