S. Sawada, S. Nomura, S. Fujii and I. Yoshida: Phys. Rev. Letters 1 (1958) 320.
See, for example, the review article by K. Hamano, S. Hirotsu and S. Sawada: Butsuri 28 (1973) 253 [in Japanese] and references cited therein.
J. W. Sidman: J. Amer. Chem. Soc. 79 (1957) 2669.
S. Makishima, T. Tomotsu, M. Hirata, S. Hayakawa, K. Hasegawa, R. Kambe and S. Shionoya: Luminescence of Organic and Inorganic Materials, ed. H. P. Kallman and G.M. Spruch (Wiley, New York, 1962) p. 443.