1. Contribution from the Laboratory of Organic Chemistry, Amsterdam Institute of Molecular Studies, Nieuwe Achtergracht 129, 1018 WS Amsterdam, The Netherlands, Physics Department, Odense University, DK-5230 Odense, Denmark, and Plant Biology and Biogeochemistry Department, Risø National Laboratory, PBK-313, Postbox 49, DK-4000 Roskilde, Denmark