Infrared Absorption of Aluminum, Copper, Lead, and Nickel at 4.2°K

The optical absorptivities of bulk samples of aluminum, copper, lead, and nickel have been determined calorimetrically at 4.2°K over the wavelength range 1-11 μ. At longer wavelengths ($\lambda >4\mathrm{} \mu$), Al, Pb, and Cu exhibit absorptivities independent of wavelength and equal to 1.20, 1.16, and 0.41%, respectively. These results are analyzed in terms of photon absorption by conduction electrons which are scattered at the metal surface and in phonon-generating collisions in the skin-depth region. The values for copper and lead are in satisfactory agreement with the predictions of the theories describing these processes; however, the predicted absorptivity value for aluminum is 30% lower than that observed. In ferromagnetic nickel, an attempt was made to study photon absorption resulting from electron scattering at magnon-generating collisions; unfortunately, the measured absorptivity varies with wavelength even at $\lambda =11\mathrm{} \mu$, suggesting the persistence of interband transitions. The 4.0% absorptivity of nickel at 11 μ is approximately twice the value estimated for intraband transitions involving surface, phonon, and magnon scattering of electrons.