1. Solid State Physics, Lund University, Box 118, 221 00 Lund, Sweden, Synchrotron Radiation Research, Lund University, Box 118, 221 00 Lund, Sweden, ESRF, B. P. 220, F-38043 Grenoble, France, Kamerlingh Onnes Laboratory, Leiden University, P.O. Box 9504, 2300 RA Leiden, The Netherlands, and Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720