1. P. Cahuzac, J. Phys. (Paris). 32, 499 (1971).
2. P. A. Bokhan, V. M. Klimkin, V. E. Prokop’ev, and V. I. Solomonov, Kvantovaya Élektron. (Moscow). 4, 152 (1977).
3. N. V. Golovchak, I. I. Garga, and L. L. Shimon, Opt. Spektrosk. 44, 23 (1978). [Opt. Spectrosc. 44, 13 (1978)].
4. R. K. Peterkop, Opt. Spektrosk. 48, 10 (1980). [Opt. Spectrosc. 48, 4 (1980)].
5. L. L. Shimon, N. V. Golovchak, I. I. Garga, and V. L. Goldovskii, Opt. Spektrosk. 55, 234 (1983). [Opt. Spectrosc. 55, 137 (1983)].