1. Reviews have been given by: Kastler, A.: J. Opt. Soc. Am.47, 460 (1957)
2. Series, G.W.: Rep. Progr. Phys.22, 280 (1959)
3. zu Putlitz, G.: Erg. d. Exakt. Natw.37, 15 (1965)
4. Budick, B.: Advance in Atomic and Molecular Physics, Vol. 3, ed. by R. D. Bates and I. Estermann, p. 73. New York: Academic Press 1967
5. zu Putlitz, G.: Atomic Physics, ed. by V.W. Hughes, B. Bederson, V. W. Cohen and F. M. J. Pichanick, p. 227. New York: Plenum Press 1969. Data are collected in Ref. 21