1. Department of Physics, AlbaNova University Center, Stockholm's Center for Physics, Astronomy and Biotechnology, Stockholm University, S 106 91 Stockholm, Sweden, Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569, and Department of Chemistry and Chemical Biology, Wright-Rieman Laboratories, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854