The spectral line shapes and the absorption frequencies of the oxygen related infrared active vibrational modes in amorphous SiO2are studied both experimentally and by simulation. Experimental data were obtained on oxides grown thermally in the temperature range from 800 to 1050 °C and on oxides deposited at 300 °C by plasma enhanced chemical deposition. The transverse optic (TO) mode centered around 1090 cm−1is found to have a line shape and peak frequency which varies significantly with film thickness while the longitudinal optic (LO) mode at 1256 cm−1is invariant. Data on both modes and on refractive index is used to demonstrate consistently that the 800 °C grown oxide is 1.6%–2.0% denser than that grown at 1050 °C. For thin oxides (<10 nm) there is evidence from both TO and LO modes that interfacial oxide is denser than the ‘‘bulk.’’ The data on deposited oxides suggest that caution must be exercised in extending the analysis to their case. ©1995 American Institute of Physics.