1. School of Chemistry, The University of Birmingham, Birmingham B15 2TT, U.K., Physics Department, Brookhaven National Laboratory, Upton, New York 11973-5000, Center for Environmental Molecular Sciences and Geosciences Department, State University of New York, Stony Brook, New York 11794, and ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, Oxon, U.K.