Cross section measurements for molecular reactions of fundamental and astrophysical relevance
Astrophysics and Astronomy
Final Report Abstract
The field of astrochemistry aims to describe the formation of stars and planets from interstellar molecular clouds. One of the most prominent charged molecules in these environments is H3+. At the typical densities and temperatures of prestellar cores, most species freeze onto dust grains. Here, H3+ become the dominant positive charge carrier in the gas and shapes the dynamical evolution of the prestellar core up to its collapse. However, the symmetric H3+ molecule cannot be directly detected by telescopes due to the absence of a rotational spectrum. Instead, the less abundant deuterated isotopologues, H2D+ and D2H+, are detected by their rotational spectrum, and the abundance of H3+ is inferred through astrochemical models. Up to now, astrochemical models were based on the assumption that H3+, H2D+, and D2H+ can turn into H2D+, D2H+, and D3+, respectively, through isotope-exchange reactions either with atomic D or with the diatomic molecules HD and D2. The latter two cases are considered to be well understood through experimental and theoretical studies, and reliable thermal rate coefficients have been implemented into astrochemical models. In contrast, the role of deuteration through collisions with atomic D remained an open question in astrochemistry up to now. Previously published theoretical cross sections showed inconsistencies and were not adopted into astrochemical models. Instead, the models assumed a temperature-independent thermal rate coefficient close to the classical value. To address this issue, I performed a series of laboratory experiments with the unique dualsource, merged fast-beams apparatus set up by the group of Dr. Daniel W. Savin at Columbia University in New York City. Co-propagating beams allowed us to measure absolute total cross sections for relative collision energies between ~10 meV to ~10 eV. By varying the internal temperature of the reacting H3+ temperature I found indications of a reaction barrier. These results motivated one of our collaborators to perform new high-level quantum ab initio calculations for the zero-point corrected energy profile along the minimum energy path of the reaction. From the combination of experimental and theoretical results I derived a temperaturedependent thermal rate coefficient, also considering possible effects of tunneling. In contrast to the current assumptions in the literature of astrochemistry, my results show clearly, that the reaction does not proceed at the relevant cold temperatures of astrochemical environments. This may affect the inferred relative abundances of different H3+ isotopologues and the astrochemical models should therefore be modified accordingly.
Publications
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Experimental and theoretical studies of the isotope exchange reaction D + H3+ → H2D+ + H. Astrophys. J.
P.-M. Hillenbrand, K. P. Bowen, J. Liévin, X. Urbain, and D. W. Savin
