Surface science investigations of the role of CO2 in astrophysical ices.

We have recorded reflection-absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) data for a range of CO2-bearing model astrophysical ices adsorbed on a graphitic dust grain analogue surface. Data have been recorded for pure CO2, for CO2 adsorbed on top of amorphous solid water, for mixed CO2:H2O ices and for CO2 adsorbed on top of a mixed CH3OH:H2O ice. For the TPD data, kinetic parameters for desorption have been determined, and the trapping behaviour of the CO2 in the H2O (CH3OH) ice has been determined. Data of these types are important as they can be used to model desorption in a range of astrophysical environments. RAIR spectra have also shown the interaction of the CO2 with H2O and CH3OH and can be used to compare with astronomical observations, allowing the accurate assignment of spectra.

Thermally induced mixing of water dominated interstellar ices.

Despite considerable attention in the literature being given to the desorption behaviour of smaller volatiles, the thermal properties of complex organics, such as ethanol (C(2)H(5)OH), 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 C(2)H(5)OH deposited on top of water (H(2)O) films of various thicknesses grown on highly oriented pyrolytic graphite (HOPG) at 98 K. Unlike many other molecules detected within interstellar ices, C(2)H(5)OH has a comparable sublimation temperature to H(2)O and therefore gives rise to a complicated desorption profile. RAIRS and TPD show that C(2)H(5)OH is incorporated into the underlying ASW film during heating, due to a morphology change in both the C(2)H(5)OH and H(2)O ices. Desorption peaks assigned to C(2)H(5)OH co-desorption with amorphous, crystalline (CI) and hexagonal H(2)O-ice phases, in addition to C(2)H(5)OH multilayer desorption are observed in the TPD. When C(2)H(5)OH is deposited beneath ASW films, or is co-deposited as a mixture with H(2)O, complete co-desorption is observed, providing further evidence of thermally induced mixing between the ices. C(2)H(5)OH is also shown to modify the desorption of H(2)O 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 C(2)H(5)OH is localized in hotter regions of the ISM, such as hot cores.