1. Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States
2. JILA, University of Colorado Boulder and the National Institute of Standards and Technology, Boulder, Colorado 80309, United States
3. Department of Chemistry, University of Colorado Boulder, Boulder, Colorado 80309, United States
4. Department of Physics, University of Colorado Boulder, Boulder, Colorado 80309, United States