The reflectances of evaporated films of Li were measured as functions of angle of incidence for photon energies between 3 and 10.7 eV, and were analyzed, by use of Fresnel’s equations, to determine the optical constants <i>n</i> and <i>k</i> for each energy. Above 6 eV, measurements with photons incident on both vacuum and substrate surfaces gave the same values of <i>n</i> and <i>k</i> within experimental limits of about ±5%. Below 6 eV, values of <i>n</i> and <i>k</i> obtained from the analysis were not reproducible; they depend in a complex way on film thickness, light polarization, and surface structure. Failure of the analysis below 6 eV is attributed to surface-roughness effects on the <i>R</i> (θ) curves, which are not accounted for in Fresnel’s equations. These effects are believed to be small above 6 eV. The dielectric function ε<sub>1</sub>, and ε<sub>2</sub> and the energy-loss function Im(-1/ε) were determined from <i>n</i> and <i>k</i> for photon energies above 6 eV. The plasma energy of Li, as determined from the condition ε<sub>1</sub> (ω)=0, is 6.7 eV; and the plasma loss peak in Im(-1/ε) is centered at 7.1 eV. In contrast to results for the heavy alkali metals K, Rb, and Cs, no evidence was found for an optical-absorption peak in ε<sub>2</sub> in the spectral region above the plasma frequency.
T. A. Callcott and E. T. Arakawa, "Ultraviolet optical properties of Li," J. Opt. Soc. Am. 64, 839-845 (1974)