June 30, 2025 to July 4, 2025
Europe/Vienna timezone

Ion–molecule reactions of acetylene inside helium nanodroplets: Formation of cationic benzene and other covalently bound hydrocarbons including PAHs

Jun 30, 2025, 3:15 PM
1h 45m
Poster Clusters, nanoparticles, biomolecules, surface interactions and self-assembly Poster Session 1

Speaker

Florian Foitzik (Universität Innsbruck)

Description

Ion-molecule reactions play a fundamental role in the chemical evolution of the Universe, driving the formation of increasingly complex organic molecules in various astrophysical environments. Reactions of ions with molecules are particularly dominant in the diffuse interstellar medium, in molecular clouds and in the photon-dominated regions, as the proportion of ionized molecules is particularly high in these parts of the Universe. On the other hand, the low temperatures in these environments also favor ion-molecule reactions - that are often characterized by very low or vanishing activation barriers - compared to neutral-neutral reactions.
One key molecular precursor of larger hydrocarbons and other organic molecules is acetylene (C$_2$H$_2$). Its widespread presence and the unique balance of stability and reactivity facilitate the bottom-up synthesis of larger carbon structures. Understanding the pathways leading from small hydrocarbons such as acetylene to bigger systems is crucial for unravelling the molecular complexity of the Universe [1].
Our group in Innsbruck investigates ion-molecule reactions inside helium nanodroplets. This matrix is ideal for studying chemical reactions of astrophysical relevance, as it is chemically inert and interacts weakly with the doped molecules. Furthermore, only barrier-free chemical reactions can take place in this environment due to its low equilibrium temperature of below 1K [2].
In this contribution, we present our findings on the formation of covalently bound molecular complexes by the sequential reaction of neutral acetylene molecules with C$_2$H$_2^+$. We report the formation of the ions C$_6$H$_6^+$, C$_8$H$_6^+$ and C$_{10}$H$_8^+$. While cationic benzene C$_6$H$_6^+$ has already been observed in experiments on electron impact ionization of neutral acetylene clusters [3], our method permits the detection of larger hydrocarbons formed by the onwards reaction of the benzene cation.

$\textbf{References}$

[1] E. Pentsak et al., ACS Earth Space Chem. 8, 798-856 (2024)
[2] S. Albertini et al., Mass Spectrom. Rev. 41, 529-567 (2022)
[3] P. O. Momoh et al., J. Am. Chem. Soc. 128, 12408-12409 (2006)

Author

Florian Foitzik (Universität Innsbruck)

Co-authors

Lisa Ganner (Universität Innsbruck) Vincent Richardson (University of Liverpool) Johannes Reichegger (Universität Innsbruck) Paul Scheier (Universität Innsbruck) Elisabeth Gruber (Universität Innsbruck)

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