1. Contribution from the Leiden Institute of Chemistry, Leiden University, Gorlaeus Laboratories, P.O. Box 9502, 2300 RA, Leiden, the Netherlands, Department of Chemistry and Centro SCS, University of Modena and Reggio Emilia, Via Campi 183, 41100 Modena, Italy, and Institute of Applied Beam Science, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki 3310-8512, Japan