1. Contribution from the Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio, 43210, Quantum Electronics Laboratory, Faculty of Physics, Adam Mickiewicz University, 85 Umultowska, Poznan 61-614, Poland, and Département de Chimie, Université de Fribourg, Chemin du Musée 9, CH-1700 Fribourg, Switzerland