1. F. I. Vergunas and N. L. Danilova, Abstracts for the Fourteenth Conference on Luminescence [in Russian], Nauka, Moscow (1965), p. 25; V. V. Kolotkov, É. M. Yashin, N. L. Danilova, and F. I. Vergunas, ibid., pp. 27, 28.
2. N. A. Vlasenko and S. A. Zyn'o, Abstracts for the Fourteenth Conference on Luminescence [in Russian], Nauka, Moscow (1965), p. 35; N. A. Vlasenko and V. S. Khomchenko, ibid., p. 35.
3. Furokoshi Mitsuo, Kenkyu Jitsuyoka Hokoku (Elect. Comm. Lab. Tech. Journ.),13, 607 (1964).
4. T. Soeya, Jap. Appl. Phys.,6, 205 (1967).
5. K. Ishiguro, T. Sasaki, T. Arai, and I. Imai, Phys. Soc. Jap.,13, 296 (1958).