1. Physik Department E20, Technische Universitaet Muenchen, D-85748 Garching, Germany, and Fritz-Haber-Institut der MPG, Department CP, Faradayweg 4-6, 14195 Berlin, Germany, Group in the Radiation Sciences, Faculté de Médecine, Université de Sherbrooke, 3001 12e avenue Nord, Sherbrooke, Quebec, Canada J1H 5N4, Department of Physics and Astronomy and Laboratory for Surface Modification, Rutgers, The State University of New Jersey, 136 Frelinghuysen Road, Piscataway, New Jersey 08854