Molecular clusters in interstellar clouds

Cluster ions of the type H-3(+).(H-2)(p), H3O+.(H2O)(q), and mixed clusters of the type H3O+.(H2O)(q)(H-2)(p) may be formed by gas-phase chemistry or by cosmic-ray-induced desorption from dust grains in dense interstellar clouds. An analysis of formation mechanisms leads to the prediction of an equilibrium abundance of H-3(+) (H-2)(p) clusters, where p = 3-4, of similar to 10(-10) n. The initial stage in the gas-phase formation of these cluster ions would be via radiative association of H-3(+) and H-2 at a rate similar to 10(-16) cm(3) s(-1). Desorption from H-2 monolayers by H-3(+) or He+ collisions with grains leads to a similar production rate for H-3(+).(H-2)(p) clusters, Such cluster ions have been observed in laboratory experiments on charged particle impact with solid H-2 layers. Cosmic-ray sputtering of adsorbed layers on dust can form cluster ions via the creation of energetic ions such H-3(+) and H2Oq. An equilibrium abundance of H-3(+).(H-2)(p) clusters, independent of cloud density, of similar to 10(-8) cm(-3) is predicted due to cosmic-ray sputtering of adsorbed Hz molecules, Sputtering of ice layers by cosmic rays should produce a range of large cluster ions H3O+.(H2O)(q) in interstellar clouds. Laboratory data on sputtering of H2O with keV He+ ions shows that clusters with q less than or similar to 50 are possible. The fragmentation of such clusters on electron-ion recombination is likely to lead to a range of neutral dusters, The abundance of such clusters, which may be considered to be a population of very small grains, is predicted to be comparable to that of dust grains. These clusters can accrete other atomic and molecular species and may constitute a gas-phase route toward grain formation in dense interstellar clouds.