1. S. E. Frish, Optical Spectra of Atoms [in Russian], Fizmatgiz, Moscow – Leningrad (1963).
2. H. E. D. Scovil and E. O. Schulz-DuBois, Phys. Rev. Lett. 2, 262 (1959).
3. E. O. Schulz-DuBois and H. E .D. Scovil, U.S. Patent, No. 3015072 (1961).
4. V. K. Konyukhov and A. M. Prokhorov, JETP Lett. 3, 286 (1966).
5. B. A. Glushko, M. E. Movsesyan, and T. O. Ovakimyan, Opt. Spectrosc. 52, 762 (1982).