1. Department of Physics & Astronomy, Stony Brook University, Stony Brook, New York 11794-3800, Departament de Química Física and IQTCUB, Facultat de Química, Universitat de Barcelona, Av. Diagonal 647, 08028-Barcelona, Spain, and Department of Chemistry, 315 S. 1400 E. RM 2124, University of Utah, Salt Lake City, Utah 84112-0850