The optical absorption spectrum of lithium metal in the range 0.5-4 eV

Abstract

Lithium mirrors have been prepared by evaporation in ultra high vacuum onto cooled fused quartz and sapphire substrates; their optical properties were determined in situ by ellipsometry. Films deposited on sapphire show an absorption edge, uncorrected for possible broadening effects, at 2.4 eV at 125 K. The position of the edge is weakly temperature-dependent. The optical mass is 1.33. Lithium mirrors deposited on fused quartz have properties varying with time after deposition. Soon after evaporation the edge at 298 K appears at 1.4 eV and the optical masa is 1.7. The films then show variable properties but eventually assume a stable behaviour similar to that obtained with films deposited on sapphire. It is tentatively suggested that the spectrum showing the 1.4 eV edge is associated with another modification of lithium. The optical data for b.c.c. lithium are not reconcilable with Ham's (1962) detailed band structure calculations. If we assume that the proposed value for the energy gap at the N point, 2.72 eV, is correct we conclude that if broadening of the optical spectra is negligible, then the Fermi surface is just in contact with the zone face.