1. JILA and Department of Chemistry, University of Colorado, 440 UCB, Boulder, Colorado 80309-0440, United States
2. JILA and NIST, University of Colorado, 440 UCB, Boulder, Colorado 80309-0440, United States
3. Institute of Physical Chemistry, Karlsruhe Institute of Technology (KIT), Fritz-Haber-Weg 2, 76131 Karlsruhe, Germany