Low-temperature measurements of the absorptivity of superconducting and normal Pb in the phonon frequency region 15-200 ${\mathrm{cm}}^{-1\mathrm{}}$ 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 $\Omega \simeq 35$ ${\mathrm{cm}}^{-1\mathrm{}}$ in the normal metal and around $\Omega +2\mathrm{}\Delta \simeq 55$ ${\mathrm{cm}}^{-1\mathrm{}}$ in the superconductor. Most features of the data can be explained by a "Golden Rule" calculation of the phonon-generation process proposed by Holstein.

• Received 16 March 1970

DOI:https://doi.org/10.1103/PhysRevLett.24.1007