The optical constants of lithium-ammonia solutions in the metallic range

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Abstract
The optical constants ε1 and ε2 of lithium-ammonia solutions in the metallic concentration range and on the metallic end of the Mott transition region have been measured using ellipsometry. The photon energies varied from 0.6 ev to 2.5 ev while the Fermi energies of the solutions are in the range 0.4 to 0.9 ev. The results in the metallic range are in qualitative agreement with the simple Drude theory but show wavelength dependent relaxation times presumably because of the relatively high photon energies. An absorption in the near IR distorts the results for the solutions near the Mott transition (e.g., ε1 shows a minimum and remains >0). A peak in Im (1/ε), as calculated from ε1 and ε2, occurs at energies within 20% of those predicted from the relaxation time and carrier density. The peak is sharp for the most concentrated solutions but broadens as the Mott transition is approached. These results support the view that these solutions are simple metals though the electron density is low relative to common metals.

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