1. Department of Physics; North Carolina State University; Raleigh NC 27695 USA
2. Department of Material Science and Engineering; University of Florida; Gainesville FL 32611 USA
3. Department of Material Science and Engineering; North Carolina State University; Raleigh NC 27695 USA
4. Institut für Physikalische Chemie; Universität Regensburg; D-93040 Regensburg Germany