The Optical Properties of Rb

The dielectric constant of Rb at -140°C has been determined in the range from 2.07 eV to 6.2 eV. The core polarizability is 0.28, the optical effective mass ratio is 1.05, and the value of | V 110 | is 0.27 eV. General properties required for the dielectric function of alkali metals under the assumption that the core levels are well separated from the conduction band are discussd by using the Nozières-Pines f-sum rule and the perfect screening sum rule. By the sum rule test, the absorption up to 10 eV is found to be accounted for with 1.25 electrons per atom. The excess value, 0.25, indicates the optical coupling between the conduction and the core electrons. The data for K and C s are also analyzed.