1. Department of Physical Chemistry and the Fritz Haber Research Center, the Hebrew University, Jerusalem 91904, Israel, Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, Department of Chemistry, Nuclear Research Center-Negev, P.O. Box 9001, Beer-Sheva 84190 Israel, and Department of Biomedical Engineering, Ben-Gurion University, P.O. Box 653, Beer-Sheva 84105, Israel