Optical Absorption of Copper and Silver at 4.2°K

Measurements of the optical absorption of copper and silver are made over the wavelength range 0.3-3.3μ at a temperature of 4.2°K where the classical mean free path of the conduction electrons is much greater than the depth of penetration of the electromagnetic field. The absorptivity is determined calorimetrically, i.e., from the temperature rise produced in the sample by the absorption of the incident radiation. In the infrared ($\lambda >1.5\mathrm{}\mu$) the absorptivity, $A$, is found to be independent of wavelength. The measured values, $A=0.0050\mathrm{}$ for copper and $A=0.0044\mathrm{}$ for silver, are in good agreement with the values obtained from theory. These results suggest that the infrared absorptivity results from absorption of photons by conduction electrons (a) during diffuse internal reflection of the electrons from the surface of the metal and (b) during phonon-generating collisions in the skin-depth layer.