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Laboratory spectroscopy techniques to enable observations of interstellar ion chemistry

Abstract

Molecular ions have long been considered key intermediates in the evolution of molecular complexity in the interstellar medium. However, owing to their reactivity and transient nature, ions have historically proved challenging to study in terrestrial laboratory experiments. In turn, their detection and characterization in space is often contingent upon advances in the laboratory spectroscopic techniques used to measure their spectra. In this Review, we discuss the advances over the past 50 years in laboratory methodologies for producing molecular ions and probing their rotational, vibrational and electronic spectra. We largely focus this discussion around the widespread H3+ cation and the ionic products originating from its reaction with carbon atoms. Finally, we discuss the current frontiers in this research and the technical advances required to address the spectroscopic challenges that they represent.

Key points

  • Molecular ions have a key role in driving chemistry in the interstellar medium because their reactions are usually exothermic and the attractive potentials that they induce increase reaction rates.

  • The laboratory spectra needed to study ions in space are challenging to collect, as ions are both difficult to produce in large quantities and highly reactive.

  • A boom in ion studies was aided by the combination of spectroscopy techniques that greatly increased the optical path length with others that modulated the signal at known frequencies while leaving noise unmodulated.

  • Action spectroscopy in cryogenic ion traps indirectly measures absorption by observing changes in counts of the mass-selected, stored ions.

  • Action spectroscopy is by far the most sensitive technique and has developed into a versatile tool in the full spectral range of molecular spectroscopy, covering rotational, vibrational and electronic motion.

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Fig. 1: Astrochemical ion–molecule reactions.
Fig. 2: NICE-OHVMS.
Fig. 3: Action spectroscopy of H3+.
Fig. 4: LIR measurements of CH3+ isotopologues.
Fig. 5: High-resolution spectra of CD2H+.
Fig. 6: Laboratory and space detection of CH+ in the infrared.
Fig. 7: Laboratory and space detection of C3H+.

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Acknowledgements

The authors thank J. Pety for providing access to observational data towards the Horsehead PDR, and J. L. Doménech and D. Neufeld for help in producing Fig. 6. Support for B.A.M. for the initial portions of this work was provided by NASA through a Hubble Fellowship (grant no. HST-HF2-51396) awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy for NASA, under contract NAS5-26555. The National Radio Astronomy Observatory is a facility of the US National Science Foundation operated under cooperative agreement by Associated Universities, Inc. S.S., O.A. and S.B. thank the Deutsche Forschungsgemeinschaft for long-term support of the development of the action spectroscopy techniques in the Cologne laboratory through numerous projects.

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Correspondence to Brett A. McGuire or Stephan Schlemmer.

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McGuire, B.A., Asvany, O., Brünken, S. et al. Laboratory spectroscopy techniques to enable observations of interstellar ion chemistry. Nat Rev Phys 2, 402–410 (2020). https://doi.org/10.1038/s42254-020-0198-0

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