1. Department of Physics; University of Rome La Sapienza; P.le A. Moro 2 00185 Rome Italy
2. Paul Scherrer Institute; SwissFEL; 5232 Villigen-PSI Switzerland
3. INFN-LNF; via E. Fermi 40 00044 Frascati Italy
4. RICMASS; Rome International Center for Materials Science Superstripes; Via dei Sabelli 119A 00185 Rome Italy
5. CNR - Istituto Struttura della Materia and Elettra-Sincrotrone Trieste; Basovizza Area Science Park 34149 Trieste Italy
6. State Key Laboratory and Institute of Elemento-Organic Chemistry; Collaborative Innovation Center of Chemical Science and Engineering (Tianjin); Key Laboratory of Functional Polymer Materials and the Centre of Nanoscale Science and Technology; Institute of Polymer Chemistry; College of Chemistry; Nankai University; Tianjin 300071 China