Multiphonon Absorption in NaF

The infrared absorption in NaF at the two C${\mathrm{O}}_2$ laser wavelengths 10.6 and 9.3 μm was measured between 4 and 400 K. The results complement the previously measured absorption for higher temperatures, and allow for a detailed comparison with theoretical predictions. The experimental temperature dependence at 10.6 μm can be excellently fitted by a curve expected for three-phonon processes in the whole temperature range; the absorption at 9.3 μm fits a mixture of three- and four-phonon processes.