The measured optical absorption of the transition metals, for photon energies below 4 eV, is empirically separated into intraband and interband components. The wavelength dependences of the intraband components do not conform to the Drude relation. The interband components extend to low energies (0.1 to 0.4 eV), and are discussed on the basis of the available band models.
A. P. LENHAM, "Absorption of Low-Energy Photons in the Transition Metals," J. Opt. Soc. Am. 57, 473-476 (1967)