1. Fysikum, Alba Nova, and Structural Chemistry, Arrhenius Laboratory, Stockholm University, SE-10691 Stockholm, Sweden, Department of Physics, Uppsala University, P.O. Box 530, SE-75121 Uppsala, Sweden, Stanford Synchrotron Radiation Laboratory, P.O. Box 20450, Stanford, California 94309, and Departments of Chemistry and Geosciences, Princeton University, Princeton, New Jersey 08544