The infrared-reflectance spectra of silica glasses were measured and analyzed with a Kramers-Kronig transformation for a better understanding of the response of silica, especially in the high-frequency region (1000 -1300 cm '). The silica samples were prepared by the sol-gel technique and subsequently subjected to heat treatments at selected temperatures to induce the formation of structures with a variable connectivity of the silicate network. The infrared reAectivity was found to increase with heattreatment temperature. This was attributed to glass densification, resulting from condensation of Si-OH groups into Si-0-Si network bridging units. This densification mechanism was also confirmed by Raman measurements.The profiles of the high-frequency parts of the transverse-optic (TO) and longitudinaloptic (LO) spectra were found consistent with the presence of two coupled modes (AS& and AS&) for the asymmetric stretching of Si-0-Si bridges, as proposed in previous works. Along these lines, the shoulder at -1200 cm in the infrared-absorption spectrum of silica, which is particularly enhanced in the spectra of silica gels, can be understood as arising from the TO response of the AS2 mode. The LO-TO splitting of the various infrared-active modes was found to depend on heat-treatment temperature. This efFect was attributed to variations of silicate network connectivity with temperature, which is presurnably a8'ecting the range of the Coulombic forces inducing the LO-TO splitting.

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