1. Department of Chemistry, University of Leuven, Celestijnenlaan 200F, B‐3001, Heverlee‐Leuven (Belgium), Fax: (+32) (0)16327992
2. Current address: Department of Atmospheric, Oceanic and Space Sciences, 1218 Space Research Building, University of Michigan Ann Arbor, 2455 Hayward Street Ann Arbor, MI 48109‐2143 (USA)