1. Institute of Physical and Theoretical ChemistryUniversity of Würzburg Am Hubland Süd 97074 Würzburg Germany
2. Dipartimento di FisicaPolitecnico di Milano Piazza Leonardo da Vinci 32 20133 Milano Italy
3. Laboratory for Femtochemistry and Synchrotron RadiationPaul Scherrer Institut (PSI) 5232 Villigen Switzerland