1. Department of Physics, SUPA, University of Strathclyde, Glasgow G4 0NG, U.K., Institute of Physical and Theoretical Chemistry, University of Regensburg, 93040 Regensburg, Germany, Chemistry Department, Murdoch University, Murdoch, W.A. 6150, Australia, and Department of Molecular and Optical Physics, Albert-Ludwigs-Universität Freiburg, 79104 Freiburg, Germany