1. Institute of Physical and Theoretical Chemistry, University of Würzburg, D-97074 Würzburg, Germany
2. Department of Chemistry, University College London, 20 Gordon St., London WC1H 0AJ, U.K.
3. Institute of Organic Chemistry, University of Würzburg, D-97074 Würzburg, Germany
4. Laboratory for Femtochemistry and Synchrotron Radiation, Paul Scherrer Institut (PSI), CH-5232 Villigen, Switzerland