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Collision-induced dissociation of acetone molecular ions with helium as a target gas has been studied at 0.65and 0.45-eV center-of-mass collision energies. Scattering contour maps show that fragment ions, CH3CO+, are predominantly backward scattered with intensity maxima lying well outside the elastic scattering circle. This can occur only by the conversion of internal energy into translational energy, demonstrating the presence of long-lived (r > 30 µ$) excited states of the acetone molecular ion. From the observed reaction dynamics we infer that the first electronically excited state Of acetone ion has the unique characteristic that on collision it releases all the excess energy into the recoil kinetic energy of the resulting internally excited ground state ion which then rapidly fragments into CH3CO+ and CH3.

The charge-transfer reaction Ar+(N2,Ar) N+2 was studied in a crossed beam experiment at 1.73 and 4.01 eV collision energies. A weak plasma ion source was used to generate a nearly pure Ar+ (2P3/2) ion beam, which is crossed at 90° by a He-seeded supersonic jet N2 beam. Detailed analysis of the results demonstrates that the reaction proceeds cleanly to generate N+2(X Σ+g, v=1) with a distribution of rotational states. A direct mechanism, qualitatively described as electron jump with negligible momentum transfer, is followed at collision energies of 1.7 and 4 eV.