1. JILA, National Institute of Standards and Technology and University of Colorado, Department of Physics, 440 UCB, Boulder, Colorado 80309, United States
2. Material
Measurement Laboratory, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, Maryland 20899, United States
3. Arthur
Amos Noyes Laboratory of Chemical Physics, Division of Chemistry and
Chemical Engineering, California Institute of Technology, 1200 East
California Boulevard, Pasadena, California 91125, United States