Intrinsic surface phonons in amorphous silica

Abstract

We have detected intrinsic surface phonons in amorphous silica by performing Raman scattering and infrared-reflectivity measurements on samples of porous Vycor glass. Qualitative agreement of our experimental spectra with theoretical calculations provides an identification of the surface features in terms of microscopic atomic motions. We find a sharp Raman-active peak at 980 ${\mathrm{cm}}^{-1\mathrm{}}$ associated with the stretching vibration of a hydroxyl group against the surface silicon atom to which it is bonded. The corresponding wagging motion of the hydroxyl group appears in the infrared spectrum as a sharp feature at 380 ${\mathrm{cm}}^{-1\mathrm{}}$. Both the appearance of these surface features as sharp peaks in the spectra and their relative amplitudes in the Raman and infrared spectra can be explained only if the sympathetic vibrations of underlying substrate layers are taken into account. Experimental confirmation of our assignment is provided by the effects of deuteration and adsorbed ammonia molecules on the Raman-active surface modes. Our experiment is the first in which the perturbations of these surface modes due to adsorbed molecules are studied. In our examination of the effects of water adsorption on the Raman spectrum of porous Vycor, we have observed a decrease in the intensity of the bulk "defect" mode near 600 ${\mathrm{cm}}^{-1\mathrm{}}$ with increasing water coverage. We also present evidence that the intense scattering background observed in Raman experiments on silica and other oxide surfaces is primarily an intrinsic electronic surface effect.