Far-Infrared Absorptivity of Normal Lead

The Holstein-Boltzmann equation has been used to calculate the far-infrared absorptivity of normal Pb in and above the phonon-frequency region. The absorptivity exhibits a threshold at $\overline{\nu }\approx 35$ ${\mathrm{cm}}^{-1\mathrm{}}$ and weak structure at $\approx 70$ ${\mathrm{cm}}^{-1\mathrm{}}$ related to the transverse and longitudinal peak in the phonon density of states, respectively. It increases rapidly through the phonon region, eventually saturating at the Holstein value. These results agree with the recent experiments of Joyce and Richards.