1. CEA-Saclay, DSM-DRECAM-SRSIM, Bât. 466, F-91191 Gif-sur-Yvette Cedex, France; Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia V6T 1Z1, Canada; CEA-Saclay, DSM-DRECAM-SPAM, Bât. 522, F-91191 Gif-sur-Yvette Cedex, France; Gas Engineering Montrouge, Schlumberger Industries, B.P. 620-05, 50, av. Jean Jaurès, F-92542 Montrouge Cedex, France