Reappraisal of the contribution from [O-2 center dot(H2O)(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-2.(H2O)(n)](+) and O-4(+). H2O, w here n is in the range 1-5. These clusters have been studied in order to id entify reaction channels which may convert O-2(+), 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-2.(H2O)(n)](+) ion can be viewed as a half-col lision intermediate in the sequence of bimolecular hydration reactions, whi ch are thought to lead to the formation of proton hydrates. Three different methods of cluster dissociation have been investigated, unimolecular (meta stable) decay, collision-induced fragmentation, and photodissociation by vi sible laser radiation (450-690 nm). The experiments show that the intermedi ates [O-2.(H2O)(n)](+), for n in the range 2-5, preferentially dissociate t o form (H2O)(n)(+) ions, a route which is largely favored over proton hydra te formation. For the first member of the series, O-2(+). H2O both collisio nal activation and photoexcitation lead to the appearance of O-2(+) and H2O as the major fragments For the trimer, [O-2.(H2O)(2)](+), the principal ph otofragment is (H2O)(2)(+) but a significant fraction of H3O+ is also obser ved. Each of the photodissociation channels observed for O-2(+). H2O and [O -2.(H2O)(2)](+) exhibits a much wider wavelength dependency than has been o bserved in previous experiments (Smith, G. P.; Lee, L. C. J. Chem. Phys. 19 78, 69, 5393. Beyer, R. A.; Vanderhoff, J. A. J. Chem. Phys. 1976, 65, 2313 ). However, we are able to reproduce these earlier measurements by monitori ng the photodissociation of "cold" clusters in the form O-2(+). H2O . Ar an d [O-2.(H2O)(2). Ar](+). A new photodissociation cross section of (9 +/- 2) x 10(-18) cm(2) has been determined for the reaction O-2(+). H2O + h upsil on --> O-2(+) + H2O in the wavelength range 450-690 nm. Taken in conjunctio n with the solar radiation flux at 87 km, the magnitude of the correspondin g unimolecular rate constant (10.8 s(-1)) suggests that the above process i n association with "warm" ions may provide an important sink, which could e xplain the low O-2(+). H2O ion concentration observed in the ionosphere (Mc Crumb, 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-2.(H2 O)(2)](+) in conjunction-with the solar radiation flux at 87 km.