1. Department of Chemical & Nuclear Engineering, University of New Mexico, Albuquerque, New Mexico 87131; Sandia National Laboratories, Albuquerque, New Mexico 87185; Department of Chemistry, University of Aarhus, Langelandsgade 140, DK-8000 Århus, Denmark; and Laboratoire de Physique, ENS Lyon, 46, allee d'Italie, 69364 Lyon, cedex 07, France