Optical constants of rubidium and cesium from 0.5 to 4.0 eV

Measurements are reported for epsilon(1) and epsilon(2), the real and imaginary parts of the dielectric constant, of Rb and Cs at room temperature in the frequency range 0.5-4.0 eV. The experiments were performed by multiple reflection at metal-silica interfaces employing the technique used earlier on Na and K. The optical absorption of Rb shows structure which can be attributed to conventional interband transitions within the nearly free-electron model. The optical absorption of Cs also shows structure, but the threshold occurs at a lower frequency than that predicted by the nearly free-electron model. Both metals show a leveling off and a perceptible rise in the absorption at the high-frequency end of the range. The possibility of absorption due to many-body effects or transitions to d bands above the Fermi level is discussed. The results for E, are used to deduce values for the infrared optical masses which are found to be 1.16 m for Rb and 1.19 m for Cs.