1. Contribution from Applied Laserphysics & Laserspectroscopy, Physics Faculty, University of Bielefeld, Universitätsstrasse 25, 33615 Bielefeld, Germany, and Centre de Biochimie Structurale, CNRS UMR 5048 - INSERM UMR 554 - Université Montpellier I, 29 Rue de Navacelles, 34090 Montpellier, France