Low-temperature measurements of the absorptivity of superconducting and normal Pb in the phonon frequency region 15-200 cm^-1 show an onset of absorption when the photon energy is large enough to excite appreciable numbers of acoustic phonons. The onset begins aroung the transverse phonon frequency Ω35 cm^-1 in the normal metal and around Ω+2Δ55 cm^-1 in the superconductor. Most features of the data can be explained by a "Golden Rule" calculation of the phonon-generation process proposed by Holstein.
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Joyce et al. (1970) studied this question.
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