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Triatomic hydrogen, the simplest polyatomic molecule, plays a key role in the chemistry of interstellar clouds. Interestingly, its singly charged ionic form H3+ is stable, but the molecule breaks apart once neutralised. The capture of a free electron with subsequent breakup of the molecule is a very efficient mechanism, called dissociative recombination. Neutral hydrogen fragments released in this process are important reactants in the chemical networks of molecular clouds. Of particular interest are the deuterated forms H2D+ and D2H+, where one or two hydrogen nuclei (i. e. protons) are replaced by the twice-as-heavy deuteron isotope. In dense molecular cloud environments, relevant for the formation of stars and planets, deuterium-insertion can be used to trace organic chemistry. Thus, a detailed knowledge of the probabilities of dissociative recombination (so-called rate coefficients) for deuterated triatomic hydrogen ions is crucial for our understanding of deuterium enrichment in star-forming regions.
In order to extract the desired information under low-temperature interstellar conditions, and to get a detailed comparison with theoretical calculations, the molecular ions have to be prepared in their lowest quantum states of internal degrees of freedom, such as vibration and rotation. While the vibrational motion is almost frozen at room temperature already, the molecular rotation needs a cryogenic environment in order to relax into its lowest states. Furthermore, the molecules created in an ion source have to be stored sufficiently long to cool down from an initial temperature of a few thousand kelvin, before the actual measurement can be started. For D2H+ stored in the CSR, model calculations show that about 80% of the ions are in the rotational ground states within 15 minutes. This corresponds to the conditions in interstellar clouds, with a typical temperature of 50 K.
The stored molecular ions interact with a nearly monoenergetic electron beam in one of the straight sections of the CSR. The relative ion-electron velocity can be tuned to the regime typical for an interstellar environment. Neutral fragments from the molecular breakup are detected in the forward direction downstream of the electron target. About 1% of the initially injected ions remain in the ring after 1000 s of cooling. For each measurement cycle of about 20 minutes, the recombination rate is determined at the beginning as well as after cooling.
As a predicted effect of internal cooling an overall decrease of the low-energy rate coefficient by a factor of 3 to 6 was observed. In addition, the dominance of the lowest rotational states can be verified by the data. Comparison with theoretical rate coefficients calculated by physicists from the University of Central Florida and Purdue University show a very good agreement in the range of corresponding kinetic temperatures below 1000 K down to 100 K. Previous data derived from room temperature studies show up to a factor 3 higher rates caused by excited rotational states. Remaining discrepancies at even lower temperatures may be caused by intrinsic uncertainties in the ab initio calculations.
The new experimental data for cold D2H+, exploiting the unique features of the CSR to their full extent, indicate an enhanced survival probability of D2H+ ions with respect to the dissociative recombination process in interstellar environments, which is of great relevance for deuterium enrichment in star forming regions. It also provides benchmark tests for the theoretical description to be applied to H3+ ions, which are very difficult to cool owing to their lack of a permanent dipole moment. The results of a complementary study for H2D+ will be published soon and new attempts are currently under way to tackle the challenge of cooling H3+.
Original publication:
Electron recombination of rotationally cold D2H+ ions
A. Znotins, A. Faure, C. H. Greene, M. Grieser, F. Grussie, L. W. Isberner, Á. Kálosi, V. Kokoouline, D. Müll, D. Paul, M. Pezzella, D. W. Savin, S. Schippers, J. Tennyson, A. Wolf, O. Novotný and H. Kreckel
Nature Communications 16, 7738 (2025). DOI: 10.1038/s41467-025-62734-6
Weblinks:
The Cryogenic Storage Ring CSR
Division "Stored and Cooled Ions" (Klaus Blaum)
