1. University of Basel, Department of Physics, Klingelbergstrasse 82, Basel CH 4056, Switzerland
2. Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, Bern, CH 3012, Switzerland
3. School of Engineering, University of Warwick, Coventry, U.K.
4. Department of Physics, Lancaster University, Lancaster, U.K.