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Molecular Clusters in Interstellar Clouds

Cluster ions of the type H+3 ∙ (H2)p, H3O+ ∙ (H2O)q, and mixed clusters of the type H3O+ ∙ (H2O)q(H2)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 ∙ (H2)p clusters, where p = 3-4, of ~10-10 n. The initial stage in the gas-phase formation of these cluster ions would be via radiative association of H+3 and H2 at a rate ~10-16 cm3 s-1. Desorption from H2 monolayers by H+3 or He+ collisions with grains leads to a similar production rate for H+3 ∙ (H2)p clusters. Such cluster ions have been observed in laboratory experiments on charged particle impact with solid H2 layers. Cosmic-ray sputtering of adsorbed layers on dust can form cluster ions via the creation of energetic ions such H+3 and H3O+. An equilibrium abundance of H+3 ∙ (H2)p clusters, independent of cloud density, of ~10-8 cm-3 is predicted due to cosmic-ray sputtering of adsorbed H2 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 ≲ 50 are possible. The fragmentation of such clusters on electron-ion recombination is likely to lead to a range of neutral clusters. 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.