AP-Seminare

Reactions of H3+ deuteration for Astrochemistry

by Pierre-Michel Hillenbrand (GSI, Darmstadt)

Europe/Berlin
KBW Lecture Hall - Side Room (GSI)

KBW Lecture Hall - Side Room

GSI

Description
The H3+ molecule and its isotopologues, H2D+ and D2H+, are prominent molecular ions in the field of astrochemistry, which aims to describe the formation of stars and planets from interstellar molecular clouds. In particular, at temperatures of ~20 K typical for prestellar cores, H3+ and its isotopologues become the dominant positive charge carriers in the gas. The symmetric molecules H3+ and D3+ have no pure rotational spectrum and cannot be excited at prestellar core temperatures. Therefore, observations of H2D+ and D2H+ are used to probe prestellar cores and to infer the abundance of H3+. However, this requires understanding the chemistry that forms and destroys these molecules. Deuteration of H3+, H2D+, and D2H+ forming H2D+, D2H+ and D3, respectively, can occur either in collisions with atomic D or with the diatomic molecules HD and D¬2. The latter two cases are considered to be well understood through experimental and theoretical studies. In contrast, the role of deuteration through collisions with atomic D remained an open question in astrochemistry up to now. To address this issue, we have carried out laboratory measurements for the three H¬3+ isotopologues that undergo isotope exchange reactions with atomic D. We have used a dual-source, merged fast-beams apparatus, which enables us to study reactions of neutral atoms and molecular ions. Co-propagating beams allow us to measure absolute total cross sections for relative collision energies ranging from ~10 meV to ~10 eV. In addition, we performed new calculations for the zero-point-energy corrected energy profile and the shape of the potential energy barrier. From the combination of experimental and theoretical results, we derived thermal rate coefficients over the temperature range relevant for astrochemical models [1]. Our results have also motivated new theoretical calculations using the ring polymer molecular dynamics (RPMD) method [2]. [1] P.-M. Hillenbrand et al., Astrophys. J. 877, 38 (2019) [2] N. Bulut et al., J. Phys. Chem. A 123, 8766 (2019)