1. Division of Quantum Chemistry and Physical Chemistry, Department of Chemistry, KULeuven, Celestijnenlaan 200F, B-3001 Heverlee, Belgium, Centre for Surface Chemistry and Catalysis, Department of Interphase Chemistry, KULeuven, Kasteelpark Arenberg 23, B-3001 Heverlee, Belgium, and Belgian Institute for Space Aeronomy, Avenue Circulaire 3, B-1180 Brussels, Belgium