Molecular Clusters in Interstellar Clouds

Cluster ions of the type H + 3 (H 2 ) p , H 3 O + (H 2 O) q , and mixed clusters of the type H 3 O + (H 2 O) 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 ~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 ~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 H 3 O + . An equilibrium abundance of H + 3 (H 2 ) p clusters, independent of cloud density, of ~10 -8 cm -3 is predicted due to cosmic-ray sputtering of adsorbed H 2 molecules. Sputtering of ice layers by cosmic rays should produce a range of large cluster ions H 3 O + (H 2 O) q in interstellar clouds. Laboratory data on sputtering of H 2 O 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.