1. Peik, E., Hollemann, G., Walther, H.: Abstract D6, Thirteenth International Conference on Atomic Physics, Munich, 1992
2. Dehmelt, H.G.: IEEE Trans. Instrum. Meas. IM-31, 83 (1982)
3. Paschen, F., Campbell, J.S.: Ann. Phys.31, 29 (1938)
4. Peik, E., Hollemann, G., Walther, H., Hannaford, P.: (unpublished)
5. Peik, E., Hollemann, G., Walther, H.: Private communication