Formation of complex molecules in translucent clouds: Acetaldehyde,
  vinyl alcohol, ketene, and ethanol via nonenergetic processing of C2H2 ice
  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.
