1. Institut des Sciences Moléculaires d’Orsay, UMR8214 Univ Paris-Sud et CNRS, Bat. 350, F-91405 Orsay Cedex, France, Service Photons Atomes & Molécules, CEA IRAMIS, Service des Photons, Atomes et Molécules, Saclay, Bat. 522, F-91191 Gif-sur-Yvette, France, and Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255