Any absorption of radiation followed by nonradiative transitions increases the temperature of the excited medium. The measurement of the sample temperature as a function of the excitation wavelength is known as calorimetric absorption spectroscopy (CAS), and its sensitivity increases with decreasing temperature. We describe here a sample/thermometer arrangement allowing quantitative CAS measurements in the mK range using a3He/4He dilution refrigerator for the detection of nonradiative processes in solids with an extreme sensitivity. The application of CAS in the mK range is demonstrated by measurements on Ni-doped CdS crystals. The determination of the quantum efficiency of inner impurity transitions becomes now possible, leading to new information on the relaxation and recombination processes of the excited impurity ion. © 1992.
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