1. Departamento de Quimica Fisica, Universidad de Murcia, 30071 Murcia, Spain, Institut de Biologie Structurale, UMR 5075 CEA-CNRS-UJF, 41 rue Jules Horowitz, 38027 Grenoble Cedex 1, France, Institute of Physics, St. Petersburg University, Ulianovskaya Street 1, 198504 St. Petersburg, Russia, and CERMAV-CNRS (Affiliated with Université Joseph Fourier), BP 53, 38041 Grenoble cedex 9, France