Longitudinal Optical Vibrations in Glasses: GeO2and SiO2

We show that longitudinal optical (LO) vibrational modes account for several previously unexplained peaks in the Raman spectra of vitreous Ge${\mathrm{O}}_{2}$ and Si${\mathrm{O}}_{2}$. Identification of LO modes in these and other glasses reduces the complexity of the spectra that must be explained by simple structural models. It also follows that long-range (Coulomb) forces should be included in complete theories of the vibrational properties of many glasses.