1. IBM Thomas J. Watson Research Center, Yorktown Heights, New York 10598
2. National Bureau of Standards, Washington, D. C. 20234
3. II. Institut fur Experimentalphysik, Universitát Hamburg, 2000 Hamburg 50 Hamburger Synchrotron Strahlungs Labor HASYLAB, Deutsches Elektronen Synchrotron DESY, 2000 Hamburg 52, Federal Republic of Germany