Energy transfer, scattering and dissociation in ion atom collisions: CO2+/Ar

Collision-induced dissociation (CID) and nondissociative scattering of CO2+ ions following collision with a supersonic molecular beam of argon has been studied at low collision energies by crossed-beam tandem mass spectrometry. The center-of-mass (c.m.) velocity contour diagram at 23.8 eV collision energy showed that the scattering of CO2+ ions have two energetically distinct components: elastic collisions at smaller angles in which momentum exchange apparently involves Ar/O repulsive interactions and inelastic collisions at larger angles in which internally excited CO2+ ions recoil from the two-body CO2+/Ar c.m. The most probable energy transfer in the inelastic process is 4.8±0.5 eV, just below the lowest dissociation threshold. The CID processes at the same collision energy leading to fragment ions, CO+ and O+, show similar characteristics. CID occurs via both spectator knock-out and two-body collisions that result into two distinct scattering patterns. The energy transfers for the two pathways for O+ fragment ions are 4.7±0.5 eV for knock-out collisions and 7.6±0.5 eV for the two-body inelastic recoil collision mechanism. It is suggested that CID for O+ via the latter process must involve an electronic state higher than the C state and proceed via curve crossing. Energy transfers for CO+ fragment ions via the two corresponding processes are 5.7±0.5 eV and 7.6±0.5 eV, respectively, clearly suggesting similar mechanisms for energy transfer and dissociation for this CID process also. It is suggested that the bent geometry of the CO2+ ions may be an important factor in promoting two distinct mechanisms.