Single crystals of MgO have been additively colored by heating them in Mg vapor at temperatures from 1600 to 1800'C and pressures up to 4000 Torr. The F+ and F' optical absorption bands (due to anion vacancies containing one and two electrons, respectively) were observed to be extremely similar but distinguishable, and were observed at liquid-nitrogen temperature to peak at 4.96 and 5.03 eV, respectively.The oscillator strength for the F' center is estimated to be 0.70&0.05. A photoconversion process F+~F' was studied. The luminescence bands for these centers were observed at 3.1 and 2.4 eV, respectively. The density of F' centers 1' produced by the additive coloring process was observed to be related to the density of atoms in the vapor $«by the equation EF =1270K«e @'~~, where Q=1.53 eV. The temperature dependence of these two bands was measured and is discussed.

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The electron affinities of SrSe, SrTe, CaSe, and CaTe have been determined using measurements of the temperature dependences of the electrical conductivity and the thermionic emission. The values obtained were 1.77,2.40,2.32, and 3.53 eV, respectively. Using these values together with previously obtained values for BaO, BaS, BaSe, BaTe, and CaO and a correlation with the lattice spacings, estimates of the electron affinities of SrO, SrS, CaS, MgO, MgS, and MgSe were obtained by extrapolation.

A critical review of the procedures used in the measurement and interpretation of thermionic emission with particular reference to nonmetallic cathodes is presented. Definitions are proposed for the terms ``true work function,'' ``effective work function,'' and ``Richardson work function.'' The use of Schottky plots is criticized and the advantages of effective work function plots over the conventional Richardson plots are discussed. Factors relating to the interpretation of the thermionic emission constants which are discussed include reflection coefficients, nonuniform true work functions, temperature dependence of the electron affinity, and temperature dependence of the Fermi energy.