Vibrational modes and local order in permanently densified silica glasses: Femtosecond and Raman spectroscopy study

The D₁ and D₂ vibrational ring modes of normal and densified silica are investigated using a time resolved pump-probe technique and spontaneous Raman spectroscopy. The vibrational response function of both techniques is derived and compared to the experimental results obtained on three samples of silica of increasing density. Comparison of the amplitudes of the time- and frequency-domain responses reveals a much larger increase in concentration of three-membered rings as compared to four-membered ones upon densification. In addition, measurements of the Raman-scattering intensity in SiO₂ also provide a mean to identify bending motions and to estimate the variation of the average Si-O-Si bond angle in densified samples.