1. Department of Physics, Umeå University, 901 87 Umeå, Sweden, Ion Cyclotron Resonance Program, National High Magnetic Field Laboratory, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310-4005, Department of Chemistry, Rice University, Houston, Texas 77251, and Physics Department, Fribourg University, Pérolles, CH-1700 Fribourg, Switzerland