1. BESSY, Albert-Einstein Str. 15, D-12489 Berlin, Germany, Department of Pharmacy, Uppsala University, P.O. Box 580, SE-751 23 Uppsala, Sweden, Department of Physics, Uppsala University, Box 530, SE-751 21 Uppsala, Sweden, Theoretical Chemistry, Royal Institute of Technology, Teknikringen 36, SE-100 44, Stockholm, Sweden, and Hefei National Laboratory of Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China