[1] Univ Florence, Dept Chem, Florence, Italy
[2] Iowa State Univ Sci & Technol, Ames Lab, Ames, IA 50011 USA
[3] Iowa State Univ Sci & Technol, Dept Phys & Astron, Ames, IA 50011 USA
[4] Univ Modena, Dept Chem, I-41100 Modena, Italy
[5] INFM, Unita Pavia, Dept Phys A Volta, I-27100 Pavia, Italy