1. Contribution from the Laboratoire de Physique Quantique, UMR 5626, IRSAMC, Université Paul Sabatier, 118 Route de Narbonne, 31064 Toulouse Cedex 4, France, Chemistry Department and Chemical Sciences Division of Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720, and LSDSMS, UMR 5636 Université Montpellier 2, 34095 Montpellier Cedex 5, France