State selected reactions of krypton ions with methane

Using a photo-electron product-ion coincidence method, the reactions of Kr(P-2(1/2,3/2)) ions with methane molecules are studied in the relative kine tic energy range between 0.1 and 1 eV. The relative total cross-sections an d the branching ratios for production of CB4+ and CH3+ are determined selec tively for the two krypton ions. Using available absolute reaction rates fo r unselected krypton ions, absolute selective rates are obtained. In additi on, a simple model is presented that describes the reactions quantitatively in terms of two distinct mechanisms: (1) reactions via "close collisions", and (2) reactions via charge exchange in "distant collisions". Based on th is model, evidence for breakdown of the "orbiting transition state theory" for the description of the close collision complex at collision energies ab ove about 0.4 eV, is obtained. (C) 2000 Elsevier Science B.V. All rights re served.