1. Department of Chemistry and Biochemistry, UCLA, Los Angeles, California 90095
2. Finnish Institute of Occupational Health, Laajaniityntie 1, FIN-01620 Vantaa, Finland
3. Department of Applied Physics, University of Kuopio, P.O. Box 1627, FIN-70211 Kuopio, Finland
4. Department of Physics, University of Helsinki, P.O. Box 9, FIN-00014 Helsinki, Finland