Collision-induced intramolecular energy transfer and dissociation of acetone molecular ion

Abstract

Two crossed-beam instruments with different geometry and with low pressure and differentially pumped ion sources were used to investigate collision-induced dissociation (CID) of acetone molecular ions activated by He and Ar collisions over the kinetic energy range 0.45–10 eV (center of mass). Long-lived electronically excited state(s) which back-scatter superelastically with conversion of 2.2±0.3 eV of internal energy into translational energy dominate the low collision energy (Erel <2 eV) CID of the acetone ion. This energy release matches the X←A electronic excitation energy difference, suggesting a very efficient E→T energy transfer mechanism for this system. The location of the intensity maximum for this mechanism on the relative velocity vector (fully rebound peak) indicates that small impact parameter ‘‘head-on’’ collisions trigger this energy transfer process. A curve-crossing mechanism is suggested to rationalize these results. At collision energies of 6 eV and higher the superelastic scattering mechanism is no longer observed. Translationally endothermic forward (but nonzero angle) scattering takes over as the predominant CID mechanism. The dynamics are consistent with the hypothesis that the endothermic CID channel proceeds on the electronically excited surface and involves the efficient T→E excitation of ground state acetone ions.