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.

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Journal J. Phys. B
Strömholm, C, Danared, H, Larson, A, Larsson, M, Marian, C, Rosén, S, … van der Zande, W. J. (1997). Imaging spectroscopy of recombination fragments of OH+. J. Phys. B, 30, 4919–4933.