Optical Absorption in the Alkali Metals: Detailed Calculations

The results of detailed computations of the optical absorption in alkali metals are presented. Nettel's formalism is used to calculate the phonon-assisted processes and modifications are made to allow approximately for the Debye-Waller factor, multiphonon terms, anharmonic effects, and the "optical pseudopotential." The direct interband absorption is also incorporated, and the validity of the classical ${\omega }^{-2\mathrm{}}$ frequency dependence for the Drude absorption is discussed. Results are presented as a function of temperature, form factor, and pseudopotential coefficient $V_{110}$. Smith's data for absorption in sodium are found to be consistent with $V_{110}\left(0\mathrm{}{}_{}{}^{\circ }{}_{}{}^{}\mathrm{K}\right)=0.28\mathrm{}$ eV. The data for potassium and rubidium are also consistent with the model but the data for cesium and lithium are not.