1. JILA, University of Colorado Boulder and National Institute of Standards and Technology, Boulder, Colorado 80309, United States
2. Department of Physics, University of Colorado Boulder, Boulder, Colorado 80309, United States
3. Department of Chemistry, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States
4. Materials Research Laboratory, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States
5. Department of Chemistry, University of Colorado Boulder, Boulder, Colorado 80309, United States