1. Lavín, C., Almaraz, M. A., Martín, I.: Z. Phys. D34, 143 (1995)
2. Denne, B., Hinnov, E.: Phys. Scr.35, 811 (1987).
3. Ivanov, L.N., Ivanova, E.P., Aghtsky, M: Phys. Rep.164, 324 (1988)
4. Seaton, M.J.: In: Advances in atomic and molecular physics, vol. 32. Bates, D.E., Bederson, B. (eds.) p. 295. New York: Academic Press 1994
5. Curtis, L.J.: Phys. Scr.35, 805 (1987)