1. Hesse, J., Oberlack, U.: Sec. Europhys. Study Conf. on Atomic and Molec. Physics of Ionized Gases, Innsbruck 1974
2. Williams, J.F., Dunbar, D.N.F.: Phys. Rev.149, 62 (1966)
3. Fedorenko, N.V.: Zh. Tekh. Fiz.24, 769 (1954)
4. McClure, G.W.: Phys. Rev.130, 1852 (1963)
5. Lange, G., Huber, B., Wiesemann, K.: J. Phys. E9, 734 (1976)