Photocleavage Of Aliphatic C–C Bonds In The Interstellar Medium

By Keith Cowing
Status Report
March 14, 2024
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Photocleavage Of Aliphatic C–C Bonds In The Interstellar Medium
a) IR spectra of C6H14 at 14 K both as-deposited and after a UV fluence of 9.8 × 1018 ph cm−2 . The position of the most prominent absorption bands is indicated in the figure. b) Difference spectrum between the UV-treated and the as-deposited spectra. — astro-ph.GA

Ultraviolet (UV) processing in the insterstellar medium (ISM) induces the dehydrogenation of hydrocarbons. Aliphatics, including alkanes, are present in different interstellar environments, being prevalently formed in evolved stars; thus, the dehydrogenation by UV photoprocessing of alkanes plays an important role in the chemistry of the ISM, leading to the formation of unsaturated hydrocarbons and eventually to aromatics, the latter ubiquitously detected in the ISM.

Here, through combined experimental results and ab-initio calculations, we show that UV absorption (mainly at the Ly-α emission line of hydrogen at 121.6 nm) promotes an alkane to an excited Rydberg state from where it evolves towards fragmentation inducing the formation of olefinic C=C bonds, which are necessary precursors of aromatic hydrocarbons.

We show that photochemistry of aliphatics in the ISM does not primarily produce direct hydrogen elimination but preferential C-C photocleavage. Our results provide an efficient synthetic route for the formation of unsaturated aliphatics, including propene and dienes, and suggest that aromatics could be formed in dark clouds by a bottom-up mechanism involving molecular fragments produced by UV photoprocessing of aliphatics.

Guillermo Tajuelo-Castilla, Jesús I. Mendieta-Moreno, Mario Accolla, Jesús M. Sobrado, Sofia Canola, Pavel Jelínek, Gary J. Ellis, José Ángel Martín-Gago, Gonzalo Santoro

Subjects: Astrophysics of Galaxies (astro-ph.GA); Chemical Physics (physics.chem-ph)
Cite as: arXiv:2403.08452 [astro-ph.GA] (or arXiv:2403.08452v1 [astro-ph.GA] for this version)
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Submission history
From: Gonzalo Santoro
[v1] Wed, 13 Mar 2024 12:06:53 UTC (5,706 KB)
Astrobiology, Astrochemistry

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