1. Schieder, R., Walther, H., Wöste, L.: Opt. Comm.5, 337 (1972)
2. Ruff, G.A., Hercher, M., Pike, H.A.: Bull. Am. Phys. Soc.16, 1340 (1971)
3. Walther, H.: Proceedings of the Conference on Laser Spectroscopy Vail. Colorado, June 1973
4. Hartig, W., Walther, H.: Appl. Phys.1, 171 (1973)
5. Hartwig, H., Schieder, R., Walther, H.: to be published