1. Department of Physics and Chemistry, University of Palermo, Viale delle Scienze, 90128 Palermo, Italy
2. LCLS, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA
3. CNRS - Institut de Biologie Structurale, Grenoble 38044, France
4. Department of Physics, UMR UR1-CNRS 6251, University of Rennes 1, Rennes, France