1. Contribution from the Department of Physics and Measurement Technology, Linköping University, S-58183 Linköping, Sweden, Service de Chimie des Matériaux Nouveaux, Centre de Recherche en Electronique et Photonique Moléculaires, Université de Mons-Hainaut, Place du Parc 20, B-7000 Mons, Belgium, and Department of Chemistry, The University of Arizona, Tucson, Arizona 85721-0041