1. Department of Physics & Astronomy, Rice University, Houston, Texas 77251, European Synchrotron Radiation Facility, 6 rue Jules Horowitz, BP 220, F-38043 Grenoble Cedex 9, France, Institut Laue-Langevin, 6 rue Jules Horowitz, BP 156, 38042 Grenoble Cedex 9, France, and National Synchrotron Light Source, Brookhaven National Laboratory, Upton, New York 11973