Imaging spectroscopy of recombination fragments of OH+

Dissociative recombination of vibrationally cold OH⁺ has been studied at the ion storage ring CRYRING, with an accurate measurement of the kinetic energy of the recombination fragments. When the incident electron energy exceeds 1.64±0.05 eV, fragments are observed with a kinetic energy of only 120±50 meV. The energetics suggest that this low-energy channel derives from either recombination of the X ₃S^- state or recombination of the metastable a ¹D state. High-quality ab initio calculations have been used to obtain the a ¹D state vibrational relaxation time (10 ms), excitation energy (2.25 eV) and radiative lifetime (0.030 s). The a ¹D dissociative recombination cross section has been obtained in a model calculation. The results imply that the a ¹D state has too small a recombination cross section and too short a radiative lifetime to explain the experimental results, which were obtained after 15 s of storage in CRYRING. We conclude that the 120 meV channel is due to recombination of OH⁺ X ³S^- into the O(³P) + H(n = 2) asymptotic limit. Possible mechanisms are discussed.