1. Technische Physik, Universität des Saarlandes, D 66041 Saarbrücken, Germany, Facultad de Física, Pontificia Universidad Católica de Chile, Santiago 22, Chile, Department of Physics and Astronomy and University of Missouri Research Reactor, University of Missouri-Columbia, Columbia, Missouri 65211, and Department of Chemistry, Technical University of Denmark, IK 207 DTU, DK-2800 Lyngby, Denmark