1. University of Innsbruck, Institute for Ion Physics and Applied Physics, Innsbruck, Austria
2. Instituto de Física Fundamental, C.S.I.C., Madrid, Spain
3. Dipartimento di Chimica, Biologia e Biotecnologie, Università di Perugia, Perugia, Italy
4. Departamento de Física and IUdEA, Universidad de La Laguna, La Laguna, Tenerife, Spain
5. Department of Physics, University of New Hampshire, Durham, NH 03824, USA