Astrophysical Ion Chemistry in Molecular Environments

Astrophysical ion chemistry concerns the formation, transformation and destruction of charged species within the diverse molecular milieus of space, from cold interstellar clouds to photon-dominated regions around young stars. Ion–molecule reactions drive much of the chemical evolution in such environments, enabling the synthesis of complex organic and inorganic molecules under conditions of extreme temperature and density. Key processes include radiative association, charge exchange, proton transfer and dissociative recombination. The presence of ions such as H3+, HeH+ and various protonated noble-gas hydrides shapes the cooling rate of gas, the ionisation balance and the pathway to molecule formation. Observations of rotational and vibrational transitions via radio, millimetre and infrared spectroscopy yield direct insight into ionic abundances and reaction networks. Laboratory experiments and high-level quantum-chemical calculations provide critical data on reaction rates, potential-energy surfaces and spectroscopic signatures, allowing astrochemical models to reproduce observed molecular distributions in star-forming regions, planetary nebulae and the diffuse interstellar medium. Understanding ion chemistry in molecular environments underpins our knowledge of cosmic chemical complexity, the emergence of prebiotic species and the thermal history of galaxies.

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Astrophysical Ion Chemistry in Molecular Environments publication trend

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Technical terms

Radiative association: A process where two species collide and form a bound molecule by emitting a photon to carry away excess energy.

Dissociative recombination: An ion–electron reaction yielding neutral fragments, often a key destruction route for molecular ions in space.

Protonated noble-gas hydride: A cationic complex consisting of a noble-gas atom bonded to a proton, exemplified by NgH+ (Ng = He, Ne, Ar).

Microsolvation: The stepwise solvation of a central ion by individual solvent (or host) atoms or molecules, leading to discrete cluster formation.

Anharmonic simulation: A computational approach that accounts for deviations from ideal harmonic vibrations when predicting spectral features.

Interstellar medium (ISM): The tenuous mixture of gas, dust and cosmic rays filling the space between stars, where astrochemical processes occur.

References

1. Microsolvation of a Proton by Ar Atoms: Structures and Energetics of ArnH+ Clusters. Molecules (2024).
2. Spectral Signatures of Protonated Noble Gas Clusters of Ne, Ar, Kr, and Xe: From Monomers to Trimers. Molecules (2022).
3. A Molecular Candle Where Few Molecules Shine: HeHHe+. Molecules (2020).