1. V.A. Fock Institute of Physics, St.-Petersburg State University, Ul'yanovskaya St. 1, 198504 St.-Petersburg, Russia, N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninskii Pr. 47, 119991 Moscow, Russia, Laboratoire de Réactivité de Surface, UMR 7609-CNRS, Université Pierre et Marie Curie-Paris 6, 4, Place Jussieu, 75252 Paris Cedex 05, France; and Institut Universitaire de France