1. National Institute of Lasers, Plasma and Radiation Physics, 76900 Bucharest-Magurele, Romania
2. Department of Physics, Royal Institute of Technology (KTH), S-100 40 Stockholm, Sweden
3. Laboratoire de Photophysique Moléculaire, Bât. 213, Université Paris XI, 91405 Orsay, France
4. Laboratoire de Chimie Physique, Université Paris VI, 75231 Paris Cedex 05, France