1. Department of Physics, Technical University of Munich, 85748 Garching, Germany
2. Department of Chemistry, Columbia University, New York, New York 10027, United States
3. Department of Physics, University of California Berkeley, Berkeley, California 94720, United States
4. The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
5. Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, United States