1. Department of Geosciences State University of New York, Stony Brook, New York 11794-2100, Department of Chemistry, State University of New York, Stony Brook, New York 11794-3400, Central Research and Development, DuPont Company, Inc., Experimental Station, P.O. Box 80262, Wilmington, Delaware 19880-0262, and Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8562