1. Department of Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden
2. Deparment of Chemistry and Molecular Biology, University of Gothenburg, Box 462, SE-405 30 Göteborg, Sweden
3. ISIS Facility, STFC Rutherford Appleton Laboratory, Harwell Campus, Didcot, OX11 0QX Oxfordshire, U.K.