Non-thermal desorption of complex organic molecules: Cosmic-ray
  sputtering of CH3OH embedded in CO2 ice
  Methanol ice is embedded in interstellar ice mantles present in dense
molecular clouds. We aim to measure the sputtering efficiencies starting from
different ice mantles of varying compositions experimentally, in order to
evaluate their potential impact on astrochemical models. The sputtering yields
of complex organic molecules is of particular interest, since few mechanisms
are efficient enough to induce a significant feedback to the gas phase. We
irradiated methanol and carbon dioxide ice mixtures of varying ratios with
swift heavy ions in the electronic sputtering regime. We monitored the
evolution of the infrared spectra and the species released to the gas phase
with a mass spectrometer. Methanol and 13C-methanol isotopologue were used to
remove any ambiguity on the measured irradiation products. The sputtering of
methanol embedded in carbon dioxide ice is an efficient process leading to the
ejection of intact methanol in the gas phase. We establish that when methanol
is embedded in a carbon-dioxide-rich mantle exposed to cosmic rays, a
significant fraction is sputtered as intact molecules. The sputtered fraction
follows the time-dependent bulk composition of the ice mantle, the latter
evolving with time due to the radiolysis-induced evolution of the bulk. If
methanol is embedded in a carbon dioxide ice matrix, as the analyses of the
spectral shape of the CO2 bending mode observations in some lines of sight
suggest, the overall methanol sputtering yield is higher than if embedded in a
water ice mantle. The sputtering is increased by a factor close to the dominant
ice matrix sputtering yield, which is about six times higher for pure carbon
dioxide ice when compared to water ice. These experiments are further
constraining the cosmic-ray-induced ice mantle sputtering mechanisms important
role in the gas-phase release of complex organic molecules from the
interstellar solid phase.
