Reappraisal of the Contribution from [O2.(H2O)\i n ]+ Cluster Ions to the Chemistry of the Ionosphere

Presented here are the results of a series of experiments which explore the dissociation patterns of the clusters [O(H2O)n]+ and O4+2O, where n is in the range 1-5. These clusters have been studied in order to identify reaction channels which may convert O2+, as seen in the E-region of the ionosphere, into H+(H2O)n clusters, which are the dominant ions in the lower D-region. Each [O(H2O)n]+ ion can be viewed as a half-collision intermediate in the sequence of bimolecular hydration reactions, which are thought to lead to the formation of proton hydrates. Three different methods of cluster dissociation have been investigated, unimolecular (metastable) decay, collision-induced fragmentation, and photodissociation by visible laser radiation (450-690 nm). The experiments show that the intermediates [O(H2O)n]+, for n in the range 2-5, preferentially dissociate to form (H2O)n+ ions, a route which is largely favored over proton hydrate formation. For the first member of the series, O2+2O, both collisional activation and photoexcitation lead to the appearance of O2+ and H2O as the major fragments. For the trimer, [O(H2O)2]+, the principal photofragment is (H2O)2+ but a significant fraction of H3O+ is also observed. Each of the photodissociation channels observed for O2+2O and [O(H2O)2]+ exhibits a much wider wavelength dependency than has been observed in previous experiments (Smith, G. P.; Lee, L. C. J. Chem. Phys. 1978, 69, 5393. Beyer, R. A.; Vanderhoff, J. A. J. Chem. Phys. 1976, 65, 2313). However, we are able to reproduce these earlier measurements by monitoring the photodissociation of “cold” clusters in the form O2+2Ar and [O(H2O)Ar]+. A new photodissociation cross section of (9 ± 2) × 10-18 cm2 has been determined for the reaction O2+2O + hv ? O2+ + H2O in the wavelength range 450-690 nm. Taken in conjunction with the solar radiation flux at 87 km, the magnitude of the corresponding unimolecular rate constant (10.8 s-1) suggests that the above process in association with “warm” ions may provide an important sink, which could explain the low O2+2O ion concentration observed in the ionosphere (McCrumb, J. L. Planet. Space Sci. 1982, 30, 559). A new rate constant of 2.4 s-1 has also been estimated for the photodissociation of “warm” [O(H2O)2]+ in conjunction with the solar radiation flux at 87 km.