1. Contribution from the Ecole Polytechnique Fédérale de Lausanne, Laboratoire de Spectroscopie Ultrarapide, Institut des Sciences et Ingénierie Chimiques, CH-1015 Lausanne-Dorigny, Switzerland, Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland, and Department of Inorganic Chemistry and Catalysis, Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands