Temperature Dependence of Multiphonon Infrared Absorption

Measurements of Harrington and Hass and of Barker indicate that the temperature dependence of the infrared absorption coefficient β in the n-phonon region is considerably weaker than β~T^n-1, which had been predicted for the high-temperature limit of multi-phonon absorption. This discrepancy is resolved by taking into account the temperature dependence of the phonon frequencies and the lattice constant. The agreement between the experimental and theoretical results with no adjustable parameters is good. A new evaluation of the multiphonon sums yields β~exp(-ωτ) directly, rather than as a sum on n.