1. Department of Physics; Paderborn University; Warburger Strasse 100 33095 Paderborn Germany
2. Physics Department E20; Technical University of Munich; D- 85748 Garching Germany
3. Department of Mathematics and Natural Science; University of Stavanger; Stavanger Norway
4. Institute of Semiconductor and Solid State Physics; Johannes Kepler University Linz; Altenberger Strasse 69 4040 Linz Austria
5. Institute of Organic Chemistry; Johannes Kepler University Linz; Altenberger Strasse 69 4040 Linz Austria