1. Department of Chemistry, University of California, Berkeley, and Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720-1460; Institute of Physics, Savanoriu Ave. 231, 02300 Vilnius, Lithuania, and Theoretical Physics Department, Faculty of Physics of Vilnius University, Sauletekio Ave. 9, build. 3, 10222 Vilnius, Lithuania; and Department of Chemistry and Center for Biological and Environmental Nanotechnology, Rice University, 6100 Main Street, Houston, Texas 77005