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