1. Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, Massachusetts 02138, USA
2. JILA, National Institute of Standards and Technology and Department of Physics, University of Colorado, Boulder, Colorado 80309, USA
3. Department of Chemistry, University of Colorado, Boulder, Colorado 80309, USA
4. Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712, USA