Optical spectra of alkali metal anion and ’’electride’’ films

Abstract

The optical absorption spectra of thin films produced by evaporation of methylamine from solutions which contain cryptated alkali cations and either alkali metal anions or solvated electrons have been examined over the region 4 000–40 000 cm−1. Films prepared from solutions of Na− show a strong absorption band at 15 400 cm−1, a pronounced shoulder at 18 900 cm−1, and a small but distinct peak at 24 500 cm−1. Films prepared from solutions of K−, Rb−, and Cs− have maxima at 11 900, 11 600, and 10 500 cm−1, respectively, while those prepared from solutions which contain e−solv have maxima at 7400 cm−1 independent of whether the solution is made by contacting K or Rb metals. Films made by evaporating solutions which contain Na−, K−, Rb−, and Cs− are gold colored by reflectance and blue by transmittance and are presumed to be similar to the crystalline salt Na+C⋅Na−, in which C is 2,2,2-cryptand. The principal absorption maxima occur at wavelengths very close to those of M− in the corresponding metal solutions in amines and ethers. However, the films contain no solvent but rather cryptated cations and alkali metal anions. Films made by evaporating solutions which contain M+C and e−solv are dark blue in color and strongly paramagnetic. The location of the absorption maximum and its independence of metal suggest that these films are ’’electride’’ salts in which the charge on M+C is balanced by a localized electron.