Probing the Morphology of Interstellar Ice Analogues

1. Probing the Morphology of Interstellar Ice Analogues In Memory of Rui Chen. Mark Collings School of Chemistry, University of Nottingham

2. TPD of CO/H2O System CO desorption profile is strongly dependent on the ice surface on which it was deposited. Trapping of CO only occurs when CO is deposited on porous ASW, or as a mixture. 30 mg cm-2 H2O (~ 0.35 mm at 8 K) 0.07 mg cm-2 CO

3. Summary of CO/H2O Thermal Behaviour

4. Simulation of Desorption in the CO/H2O System We developed a kinetic model of the thermal behaviour of the CO-H2O system. • The model contains 16 reaction steps. • All but 3 of the kinetic parameters were experimentally determined. • The remaining 3 were varied in the model until the model matched the experimental TPD results. Collings et al., 2003, Astron. & Space Sci., 285, 633.

5. Simulation of Desorption in the CO/H2O System The heating rate can was then slowed to values applicable during star formation. The model predicts the temperatures at which various desorption processes occur in the interstellar medium. Collings et al., 2003, Astron. & Space Sci., 285, 633.

6. Desorption of Other Species A rapid survey of the behaviour of other species desorbing from H2O reveals three categories. Water like: desorb coincidently with water. CO-like: diffuse throughout the porous structure of water ice at low temperature, showing two desorptions of trapped molecules, desorption from the H2O surface, and (if deposited as a layer) desorption of the solid species. Intermediate: show two desorptions of trapped molecules, but a limited ability to diffuse within porous H2O. Collings et al., 2004, Mon. Not. Roy. Astron. Soc., 354, 1133.

7. Comparison of CO TPD from H2O, CH3OH and mixtures. 100 L CH3OH or H2O or a 1:1 mixture of CH3OH + H2O deposited at 8 K. 10 L CO deposited at 8 K. Heating rate = 0.08 K s-1.

8. RAIR Spectroscopy of CH3OH 100 L CH3OH deposited at 8 K. CH3OH film annealed at a given temperature for 30 mins. Sample cooled to 8 K before IR scan recorded.

9. RAIR Spectroscopy of CO onH2O and CH3OH CO / H2O CO / CH3OH

10. TPD of CH3OH CH3OH deposited at 8 K. Heating rate = 0.08 K s-1 Desorption profile fits zeroth order desorption kinetics.

11. CO TPD: a probe of CH3OH morphology 100 L of CH3OH deposited at varied temperatures. 10 L of CO deposited after sample cooled to 8 K. Heating rate = 0.08 K s-1 As with H2O, the ability of CH3OH to trap CO is reduced as the deposition temperature increases.

12. Temperature Dependence of CO Trapping

13. TPD of the CO/(H2O + CH3OH) System 100 L (H2O + CH3OH) gas mixture deposited at 15 K. 5 L CO deposited at 15 K Heating rate = 0.08 K s-1

14. TPD of the CO/(H2O + CH3OH) System 100 L (H2O + CH3OH) gas mixture deposited at 15 K. 5 L CO deposited at 15 K Heating rate = 0.08 K s-1

15. Conclusions • There are strong similarities between the phase changes of H2O and CH3OH as probed by CO TPD experiments • Further investigation of desorption of the CO/(H2O + CH3OH system is required: • Desorption of co-deposited (CO + H2O + CH3OH) may prove more revealing of the phase transitions in H2O + CH3OH mixed ices. • Extension to IR spectroscopy of (H2O + CH3OH ) and (CO + H2O + CH3OH) will also help us to understand the system.

16. Acknowledgements Dr Martin McCoustra Rui Chen, Simon Green, Laura Tye, John Thrower The Leverhulme Trust, The University of Nottingham, £££ Engineering and Physical Sciences Research Council £££ (EPSRC) U.K., ORS Awards.