Association reactions with state selected ions at meV collision energies: CO+ (v=0,j) + 2 CO ? (CO) 2 + +CO

Abstract

Relative three-body association rate coefficients for the title reaction have been measured as a function of the rotational statej of the CO+ ion at collision energies below 1 meV. A significant decrease of the dimer formation has been observed whenj increases from 0 to 7 (Erot=0−13.7 meV). Multiphoton ionization (MPI) is used to create the ions in the high pressure region of a pulsed supersonic expansion of pure CO. This method allows both the rather precise determination of the rotational population of the neutrals (〈Erot〉 ∼1.5 meV) and the preparation of the ions exclusively in the vibrational ground state and in well defined rotational states. Due to its very steep energy dependence the ternary association reaction can be “switched off” within a few tenths of a millimeter using an acceleration field of 1–3 V/cm. The short effective interaction time of less than 250 ns leads to almost single collision conditions. i.e. we perform state selective studies. The kinetics and kinematics (mean kinetic energy, competition between relaxation and association) are analyzed numerically. A consistent picture results from the assumption that the ternary rate coefficient depends only on the total energy of the complex irrespective of whether its origin is rotation or translation.