  The occurrence of complex organic molecules (COMs) in the gas phase at low
temperature in the dense phases of the ISM suggests that a non-thermal
desorption mechanism is at work because otherwise, COMs should condense within
a short timescale onto dust grains. Vacuum ultraviolet photodesorption has been
shown to be much less efficient for complex organic molecules, such as
methanol, because mostly photoproducts are ejected. The induced photolysis
competes with photodesorption for large COMs, considerably lowering the
efficiency to desorb intact molecules. We investigate the sputtering efficiency
of complex organic molecules that are observed either in the ice mantles of
interstellar dense clouds directly by infrared spectroscopy (CH3OH), or that
are observed in the gas phase by millimeter telescopes (CH3COOCH3) and that
could be released from interstellar grain surfaces. We irradiated ice films
containing complex organic molecules (methanol and methyl acetate) and water
with swift heavy ions in the electronic sputtering regime. We monitored the
infrared spectra of the film as well as the species released to the gas phase
with a mass spectrometer. We demonstrate that when methanol or methyl acetate
is embedded in a water-ice mantle exposed to cosmic rays, a large portion is
sputtered as an intact molecule, with a sputtering yield close to that of the
main water-ice matrix. This must be even more true for more volatile ice
matrices, such as those that are embedded in carbon monoxide. Cosmic rays
penetrating deep into dense clouds provide an efficient mechanism to desorb
complex organic molecules. Compared to VUV photons induced by the interaction
of cosmic rays, a large portion desorb as intact molecules with a proportion
corresponding to the time-dependent bulk composition of the ice mantle, the
latter evolving with time as a function of fluence due to the radiolysis of the
bulk.
