1. Windholz, L., Umfer, C.: Z. Phys. D29, 121 (1994)
2. Windholz, L.: Appl. Phys. B59, 515 (1994)
3. Johansson, I.: Ark. Fys.15, 169 (1959)
4. Radziemski, L.J., Sansonetti, C.J.: Private communication 1994
5. Sansonetti, C.J., Engleman, R., Radziemski, L.J.: Phys. Rev. A (in press)