1. Chemical Science and Technology Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8394, Institute of National Measurement Standards, National Research Council Canada, Ottawa, Ontario K1A 0R6, Canada, Metals Division, Centro Nacional de Metrología, 76900 Querétaro, México, Optical and Radiometric Division, Centro Nacional de Metrología, 76900 Querétaro, México, and Physics Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8441