1. Institut für Ionenphysik, Leopold-Franzens Universität, Technikerstr. 25, A-6020 Innsbruck, Austria, V. Ĉermák Laboratory, J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejškova 3, 182 23 Prague 8, Czech Republic, and Department of Plasma Physics, Comenius University, Mlynska dolina, SK-842 48 Bratislava, Slovak Republic