1. Department of Chemical Physics, Lund University, Box 124, S 22100 Lund, Sweden, Department of Organic Chemistry and Biochemistry, University of Naples Federico II, Naples, Italy, Department of Physics and Nanotechnology, Aalborg University, Denmark, and Biophysics Group, Department of Physics and Astronomy, Faculty of Sciences, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam