Formation of complex organic molecules in molecular clouds:
  acetaldehyde, vinyl alcohol, ketene, and ethanol via the "energetic"
  processing of C$_2$H$_2$ ice
  The simultaneous detection of organic molecules of the form
C$_2$H$_{\text{n}}$O, such as ketene (CH$_2$CO), acetaldehyde (CH$_3$CHO), and
ethanol (CH$_3$CH$_2$OH), toward early star-forming regions offers hints of
shared chemical history. Several reaction routes have been proposed and
experimentally verified under various interstellar conditions to explain the
formation pathways involved. Most noticeably, the non-energetic processing of
C$_2$H$_2$ ice with OH-radicals and H-atoms was shown to provide formation
routes to ketene, acetaldehyde, ethanol, and vinyl alcohol (CH$_2$CHOH) along
the H$_2$O formation sequence on grain surfaces. In this work, the
non-energetic formation scheme is extended with laboratory measurements
focusing on the energetic counterpart, induced by cosmic rays penetrating the
H$_2$O-rich ice mantle. The focus here is on the H$^+$ radiolysis of
interstellar C$_2$H$_2$:H$_2$O ice analogs at 17 K. Ultra-high vacuum
experiments were performed to investigate the 200 keV H$^+$ radiolysis
chemistry of predeposited C$_2$H$_2$:H$_2$O ices, both as mixed and layered
geometries. Fourier-transform infrared spectroscopy was used to monitor in situ
newly formed species as a function of the accumulated energy dose (or H$^+$
fluence). The infrared (IR) spectral assignments are further confirmed in
isotope labeling experiments using H$_2$$^{18}$O. The energetic processing of
C$_2$H$_2$:H$_2$O ice not only results in the formation of (semi-) saturated
hydrocarbons (C$_2$H$_4$ and C$_2$H$_6$) and polyynes as well as cumulenes
(C$_4$H$_2$ and C$_4$H$_4$), but it also efficiently forms O-bearing COMs,
including vinyl alcohol, ketene, acetaldehyde, and ethanol, for which the
reaction cross-section and product composition are derived. A clear composition
transition of the product, from H-poor to H-rich species, is observed as a
function of the accumulated energy dose.
