1. Department of Physics, Göteborg University, Göteborg SE-412 96, Sweden, Department of Physics, Konkuk University, Seoul 143-701, Korea, Department of Applied Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden, Nokia Research Center, Nokia Group, P.O. Box 407, FIN-00045 Helsinki, Finland, and School of Chemistry, Edinburgh University, West Mains Road, Edinburgh EH9 3JJ, Scotland