The infra-red absorptivity of vitreous silica has been investigated in the region 100–15 cm–1 at 300 and 100 K. Above 18 cm–1 the absorptivity is independent of temperature and is undoubtedly caused by fundamental vibrations. A theory suggests that the absorptivity divided by the frequency squared should be more closely related to the density of vibrational states than should the direct spectrum. There is a maximum in this function at about 38 cm–1 which is due to the vibrations that can be considered as derived from the transverse acoustic maximum of cristobalite when it is vitrified. There is no evidence of fine structure in the infra-red spectrum, and it seems likely that the greatly increased heat capacity of vitreous silica at low temperatures over that of cristobalite is partly caused by the broadening of the transverse acoustic maximum.
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