1. Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, Molecular and Biophysics Group, Institute for Molecules and Materials, Radboud University Nijmegen, P.O. Box 9010, NL-6500 GL Nijmegen, The Netherlands, and Biophysics Group, Physics Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8443