1. Department of Physical Chemistry, Uppsala University, P.O. Box 579, S-751 23 Uppsala, Sweden, the Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Max Born Strasse 2A, D-12489, Berlin, Germany, and the Department of Physics, Freie Universität Berlin, Arnimallee 14, D-14195 Berlin, Germany