Optical Constants of Metal-Ammonia Solutions

An ellipsometric technique has been used to determine the optical constants of sodium-, lithium-, and calcium-ammonia solutions between 0.6 and 2.3 eV in the temperature range of 213 to 233 K. The sodium-ammonia data span the range of the metal-nonmetal transition. The alkali metal solutions of concentration $x\ge 8$ mole percent metal (MPM) are clearly free-electronlike, whereas, the solutions of concentration ≤ 4 MPM strongly indicate the presence of the solvated electron. In the intermediate concentration range ($4<x<\mathrm{}8\mathrm{}$ MPM), the solutions are undergoing the metal-nonmetal transition; and the species governing the optical properties cannot be unambiguously determined, but probably involve both bound and free electrons. The temperature dependence of the optical constants of concentrated ($x\ge 6$ MPM) alkali metal-ammonia solutions was found to be negligibly small, while that of less concentrated solutions was consistent with that observed in the dilute alkali metal solutions. Concentrated solutions of calcium in ammonia (>6 MPM) deviated considerably from the Drude model but still appeared to be free-electronlike, as evidenced by the lack of structure in the absorption. They differed somewhat from the alkali metal solutions of similar electron concentration.