Direct and inverse reactions of LiH+ with He(1S) from quantum calculations: mechanisms and rates

The role of small molecular cations in the chemical flow of the interstellar environments

Molecular ions have been ubiquitous in a variety of environments in the interstellar medium, from Circumstellar Envelopes to Dark Molecular Clouds and to Diffuse Clouds. Their role in the multitude of molecular processes which have been found to occur in those environments has been the subject of many studies over the years, so that we have acquired by now a complex body of data on their chemical structures, their possible function within chemical reactions and their most likely paths to formation. In the present work we review a broad range of such molecular ions, focusing exclusively on positive ions involving the smallest and simplest cations which have been either detected or conjectured as present in the interstellar medium (ISM). We therefore consider mainly molecular cations formed with components like H, H+, He and He+, atomic species which are by far the most abundant baryons in the ISM in general. Their likely structures and their roles in a variety of chemical energy flow paths are discussed and presented within the context of their interstellar environments.

Cationic complexes of hydrogen with helium

High-resolution mass spectra of helium nanodroplets doped with hydrogen or deuterium reveal that copious amounts of helium can be bound to H(+), H(2)(+), H(3)(+), and larger hydrogen-cluster ions. All conceivable He(n)H(x)(+) stoichiometries are identified if their mass is below the limit of ≈120 u set by the resolution of the spectrometer. Anomalies in the ion yields of He(n)H(x)(+) for x=1, 2, or 3, and n≤30 reveal particularly stable cluster ions. Our results for He(n)H(1)(+) are consistent with conclusions drawn from previous experimental and theoretical studies which were limited to smaller cluster ions. The He(n)H(3)(+) series exhibits a pronounced anomaly at n=12 which was outside the reliable range of earlier experiments. Contrary to findings reported for other diatomic dopant molecules, the monomer ion (i.e. H(2)(+)) retains helium with much greater efficiency than hydrogen-cluster ions.