1. JILA, University of Colorado and National Institute of Standards and Technology, Boulder, Colorado 80309-0440, United States
2. Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado
80309-0215, United States
3. Department of Physics, Lafayette College, Easton, Pennsylvania 18042, United States
4. Department of Physics, University of Colorado, Boulder, Colorado
80309-0440, United States