1. Department of Physics and Astronomy, Vrije Universiteit, De Boelelaan 1081, 1081 HV, Amsterdam, The Netherlands, Ultrafast Laser and Spectroscopy Laboratory, Materials Science Center, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands, and Institute of Biomedical and Life Sciences, Division of Biochemistry and Molecular Biology, University of Glasgow, Glasgow G12 8QQ, UK