1. KFKI Atomic Energy Research Institute, H-1525 Budapest, P.O. Box 49, Hungary
2. Research Institute for Solid State Physics and Optics, H-1525 Budapest P.O. Box 49, Hungary
3. New Mexico State University, P.O. Box 30001 MSC 3450, Las Cruces, New Mexico 88001
4. Institute for Energy, Joint Research Centre, Postbus 2 NL-1755 ZG-Petten, The Netherlands