1. e.g. Handrich, E., Kretzen, H., Lange, W., Steudel, A., Wallenstein, R., Walther, H.: Proc. Int. Conf. Opt. Pumping and Atom. Line Shape, Warszawa 1968.
2. Müller, W., Steudel, A., Walther, H.: Z. Physik183, 303 (1965).
3. Kühl, J.: Z. Physik242, 66 (1971).
4. Handrich, E., Steudel, A., Wallenstein, R., Walther, H.: Physics of one- and two-electron atoms, edited by F. Bopp and H. Kleinpoppen. North Holland Publishing Co. 1969.
5. Evans, L., Sandars, P. G. H., Woodgate, G. K.: Proc. Roy. Soc. (London), Ser. A289, 114 (1965).