1. W. M. Keck Laboratory, California Institute of Technology, Pasadena, California 91125, Department of Chemical Engineering, Environmental Engineering Program, Yale University, New Haven, Connecticut 06520-8286, School of Physics and Energy Sciences, Kyungpook National University, Daegu, 702-701, Korea, and 3M Environmental Laboratory, 3M Center, Building 260-05-N-17, Maplewood, Minnesota 55144-1000