Formation of complex molecules in translucent clouds: Acetaldehyde, vinyl alcohol, ketene, and ethanol via nonenergetic processing of C2H2 ice
Journal
Date Issued
2020
Author(s)
Chuang, K. -J.
•
FEDOSEEV, GLEB
•
Qasim,D.
•
Ioppolo, S.
•
Jäger, C.
•
Henning, Th.
•
•
Dishoeck, E. F. van
•
Linnartz, H.
Abstract
Complex organic molecules (COMs) have been identified toward high- and
low-mass protostars as well as molecular clouds, suggesting that these
interstellar species originate from the early stage(s) of starformation. The
reaction pathways resulting in COMs described by the formula C$_2$H$_\text{n}$O
are still under debate. In this work, we investigate the laboratory possible
solid-state reactions that involve simple hydrocarbons and OH-radicals along
with H$_2$O ice under translucent cloud conditions (1$\leq$A$_V$$\leq$5 and
\textit{n}$_\text{H}$$\sim$10$^3$ cm$^{-3}$). We focus on the interactions of
C$_2$H$_2$ with H-atoms and OH-radicals, which are produced along the H$_2$O
formation sequence on grain surfaces at 10 K. Ultra-high vacuum (UHV)
experiments were performed to study the surface chemistry observed during
C$_2$H$_2$ + O$_2$ + H codeposition, where O$_2$ was used for the in-situ
generation of OH-radicals. Reflection absorption infrared spectroscopy (RAIRS)
was applied to in situ monitor the initial and newly formed species. After
that, a temperature-programmed desorption experiment combined with a Quadrupole
mass spectrometer (TPD-QMS) was used as a complementary analytical tool. The
investigated 10 K surface chemistry of C$_2$H$_2$ with H-atoms and OH-radicals
not only results in semi and fully saturated hydrocarbons, such as ethylene
(C$_2$H$_4$) and ethane (C$_2$H$_6$), but it also leads to the formation of
COMs, such as vinyl alcohol, acetaldehyde, ketene, ethanol, and possibly acetic
acid. It is concluded that OH-radical addition reactions to C$_2$H$_2$, acting
as a molecular backbone, followed by isomerization (i.e., keto-enol
tautomerization) via an intermolecular pathway and successive hydrogenation
provides a so far experimentally unreported solid-state route for the formation
of these species without the need of energetic input. The kinetics of acetaldehyde reacting with impacting H-atoms leading to ketene and ethanol is found to have a preference for the
saturated product. The astronomical relevance of the reaction network introduced here is discussed.
Funding(s)
AstroFIt2 (Astronomy Fellowships in Italy 2)
Volume
635
Start page
A199
Issn Identifier
0004-6361
Ads BibCode
2020IAUS..350...77P
Rights
open.access