Near-thermo-neutral electron recombination of titanium oxide ions

While the dissociative recombination (DR) of ground-state molecular ions with low-energy free electrons is generally known to be exothermic, it has been predicted to be endothermic for a class of transition-metal oxide ions. To understand this unusual case, the electron recombination of titanium oxide ions (TiO⁺) with electrons has been experimentally investigated using the Cryogenic Storage Ring. In its low radiation field, the TiO⁺ ions relax internally to low rotational excitation (≲100 K). Under controlled collision energies down to ∼2 meV within the merged electron and ion beam configuration, fragment imaging has been applied to determine the kinetic energy released to Ti and O neutral reaction products. Detailed analysis of the fragment imaging data considering the reactant and product excitation channels reveals an endothermicity for the TiO⁺ dissociative electron recombination of (+4 ± 10) meV. This result improves the accuracy of the energy balance by a factor of 7 compared to that found indirectly from hitherto known molecular properties. Conversely, the present endothermicity yields improved dissociation energy values for D₀(TiO) = (6.824 ± 0.010) eV and D₀(TiO⁺) = (6.832 ± 0.010) eV. All thermochemistry values were compared to new coupled-cluster calculations and found to be in good agreement. Moreover, absolute rate coefficients for the electron recombination of rotationally relaxed ions have been measured, yielding an upper limit of 1 × 10^-7 cm³ s^-1 for typical conditions of cold astrophysical media. Strong variation of the DR rate with the TiO⁺ internal excitation is predicted. Furthermore, potential energy curves for TiO⁺ and TiO have been calculated using a multi-reference configuration interaction method to constrain quantum-dynamical paths driving the observed TiO⁺ electron recombination.