1. I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
2. Institut für Physikalische Chemie, Universität Mainz, 55099 Mainz, Germany
3. Dipartimento di Chimica Fisica e Inorganica, Università di Bologna, Viale del Risorgimento 4, I-40136 Bologna, Italy
4. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, United States
5. School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, United States