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 bee n studied at low collision energies by crossed-beam tandem mass spectrometr y. 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 disti nct components: elastic collisions at smaller angles in which momentum exch ange apparently involves Ar/O repulsive interactions and inelastic collisio ns at larger angles in which internally excited CO2+ ions recoil from the t wo-body CO2+/Ar c.m. The most probable energy transfer in the inelastic pro cess is 4.8 +/-0.5 eV, just below the lowest dissociation threshold. The CI D processes at the same collision energy leading to fragment ions, CO+ and O+, show similar characteristics. CID occurs via both spectator knock-out a nd two-body collisions that result into two distinct scattering patterns. T he 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 procee d via curve crossing. Energy transfers for CO+ fragment ions via the two co rresponding processes are 5.7 +/-0.5 eV and 7.6 +/-0.5 eV, respectively, cl early suggesting similar mechanisms for energy transfer and dissociation fo r 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. (C) 2001 American Institute of Physics.