1. Fysikum, AlbaNova, Stockholm University, SE-106 91 Stockholm, Sweden, Stanford Synchrotron Radiation Laboratory, P.O. Box 20450, Stanford, California 94309, U.S.A., Department of Chemistry, Princeton University, Princeton, New Jersey 08544, U.S.A., BESSY, Albert-Einstein-Strasse 15, D-12489 Berlin, Germany, and Department of Geosciences, Princeton University, Princeton, New Jersey 08544, U.S.A.