1. CPMOH, U. Bordeaux 1, 351 Cours de la Libération, 33405 Talence, France
2. LPS, ENS, 24 Rue Lhomond, 75005 Paris, France and van der Waals-Zeeman Instituut, University of Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, The Netherlands
3. Department of Chemical Physics, The Weizmann Institute of Science, Rehovot 76100, Israel