1. Department of Physics, Umeå University, 901 87 Umeå, Sweden, Ion Cyclotron Resonance Program, National High Magnetic Field Laboratory, Tallahassee, Florida 32310-4005, Physics Department, Fribourg University, Pérolles, CH-1700 Fribourg, Switzerland, and Institute of Problems of Chemical Physics, RAN, 142432, Chernogolovka, Moscow Region, Russia