1. IBM Research Division, Zurich Research Laboratory CH-8803 Rüschlikon, Switzerland Scuola Normale Superiore, I-56126 Pisa, Italy Department of Physics, University of California Los Angeles, California 90024 School of Chemistry, Physics and Environmental Science University of Sussex, Flamer, Brighton BN1 9QJ, U.K. Institute for Polymers and Organic Solids Department of Chemistry and Materials University of California, Santa Barbara, California 93106