Optical Constants of Sodium and Potassium from 0.5 to 4.0 eV by Split-Beam Ellipsometry

The optical constants of sodium and potassium have been measured at room temperature over the frequency range 0.5-4.0 eV. The measurements were made with a split-beam ellipsometer, using a null procedure devised to yield the imaginary part of the dielectric constant, ε₂, with improved reliability. The samples were thick films prepared by evaporation in ultrahigh vacuum. Experiments were performed by single reflection at free surfaces, and by both single and multiple reflections at metal-fused quartz interfaces. All measurements were in agreement; however, the most accurate values for ε₂ were obtained in the multiple reflection experiments. They showed structure as a function of frequency which could be readily attributed to conventional interband transitions associated with the $〈110〉$ reciprocal lattice vectors. The values obtained for ε₁ in both the free-surface and the metal-quartz-interface experiments were used to deduce the optical effective mass mₒₚₜ. For sodium, mₒₚₜ was found to be $1.13m$ in the infrared and $1.07m$ for wavelenghts below 0.5{μ}; the corresponding values for potassium were found to be $1.16m$ and $1.06m$.