1. Contribution from the Department of Chemistry, University of Illinois at Urbana-Champaign, 505 South Mathews Avenue, Urbana, Illinois 61801, Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, 104 South Goodwin Avenue, Urbana, Illinois 61801, and Ecole Polytechnique Fédérale de Lausanne, Institut de Physique Expérimentale, CH-1015 Lausanne, Switzerland