1. Contribution from the IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120, Lockheed Martin Missiles and Space Advanced Technology Center, 3251 Hanover Street, Palo Alto, California 94304, and Institut für Physikalische Chemie, Johannes Gutenberg-Universität, Jakob Welder-Weg 11, 55099 Mainz, Germany