The extinction coefficients, k, of sodium and potassium have been found by measuring the transmittance of vacuum-evaporated films 400-900 nm thick deposited on quartz or lithium fluoride substrates. Corrections were made for losses due to surface reflections, substrate absorption, and contamination of the exposed surface of the metal. Previously determined values of the refractive indices, n, of Na and K were used to compute the imaginary part, ∊2 = 2nk, of the dielectric constant. Above the interband energies, at 2-3 eV, ∊2 for both Na and K decreases rapidly with increasing photon energy up to the plasma energy. Beyond the plasma energy, ∊2 remains nearly constant within the limits of our measurements. The values of ∊2 for K are more than twice those for Na in the region above the plasma energy.
J. C. SUTHERLAND, R. N. HAMM, and E. T. AAIAKAWA, "Extinction Coefficient and Imaginary Part of the Dielectric Constant for Sodium and Potassium above the Plasma Energy," J. Opt. Soc. Am. 59, 1581-1583 (1969)