Thermally induced mixing of water dominated interstellar ices

Abstract

Despite considerable attention in the literature being given to the desorption behaviour of smaller volatiles, the thermal properties of complex organics, such as ethanol (C₂H₅OH), which are predicted to be formed within interstellar ices, have yet to be characterized. With this in mind, reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD) have been used to probe the adsorption and desorption of C₂H₅OH deposited on top of water (H₂O) films of various thicknesses grown on highly oriented pyrolytic graphite (HOPG) at 98 K. Unlike many other molecules detected within interstellar ices, C₂H₅OH has a comparable sublimation temperature to H₂O and therefore gives rise to a complicated desorption profile. RAIRS and TPD show that C₂H₅OH is incorporated into the underlying ASW film during heating, due to a morphology change in both the C₂H₅OH and H₂O ices. Desorption peaks assigned to C₂H₅OH co-desorption with amorphous, crystalline (CI) and hexagonal H₂O-ice phases, in addition to C₂H₅OH multilayer desorption are observed in the TPD. When C₂H₅OH is deposited beneath ASW films, or is co-deposited as a mixture with H₂O, complete co-desorption is observed, providing further evidence of thermally induced mixing between the ices. C₂H₅OH is also shown to modify the desorption of H₂O at the ASW-CI phase transition. This behaviour has not been previously reported for more commonly studied volatiles found within astrophysical ices. These results are consistent with astronomical observations, which suggest that gas-phase C₂H₅OH is localized in hotter regions of the ISM, such as hot cores.