1. Laboratoire de Photophysique Moléculaire, C. N. R. S., Bâtiment 350, Campus d’Orsay, 91405 Orsay Cedex, France
2. Department of Physics, Michigan Technological University, Houghton, Michigan 49931
3. Niels Bohr Institute for Astronomy, Physics and Geophysics, Copenhagen University Observatory, Juliane Maries vej 30, DK-2100 Copenhagen, Denmark