1. National Institute of Lasers, Plasma and Radiation Physics, P.O. Box MG-36, 76900-Magurele, Romania, Laboratoire de Chimie Physique (UMR 8000 CNRS), Université Paris-Sud, 91405, Orsay Cedex, France, and Biophysics Department, University of Bucharest, 76900-Magurele, Romania