1. Department of Chemistry, Stanford University, Stanford, California 94305-5080, Center of Mathematical Statistics, Casa Academiei Romane, Calea 13 Septembrie 13, Bucharest 5 Romania, and Institut für Experimentelle Physik, Otto-von-Guericke-Universität Magdeburg, Unversitätsplatz 2, D-39106 Magdeburg, Germany