1. H. L. Dodge Department of Physics and Astronomy; University of Oklahoma; Norman OK 73019 USA
2. Department of Physics; Southern Illinois University-Carbondale; Carbondale IL 62091 USA
3. Laboratoire Kastler Brossel; CNRS, ENS, UPMC; Case 74 F-75252 Paris France
4. Institut für Physik; Universität Augsburg; Universitätsstraße 1 D-86135 Germany