1. Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark, Department of General Physics and Spectroscopy, University of Vilnius, Sauletekio 9, b. 3, 2040 Vilnius, Lithuania, and Laser Centre and Department of Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands