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

School of Chemistry University of Nottingham TPD of CO/H 2 O 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  g cm -2 H 2 O (~ 0.35  m at 8 K) 0.07  g cm -2 CO

School of Chemistry University of Nottingham Summary of CO/H 2 O Thermal Behaviour

School of Chemistry University of Nottingham Simulation of Desorption in the CO/H 2 O System We developed a kinetic model of the thermal behaviour of the CO- H 2 O 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.

School of Chemistry University of Nottingham Simulation of Desorption in the CO/H 2 O 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.

School of Chemistry University of Nottingham Desorption of Other Species A rapid survey of the behaviour of other species desorbing from H 2 O 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 H 2 O 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 H 2 O. Collings et al., 2004, Mon. Not. Roy. Astron. Soc., 354, 1133.

School of Chemistry University of Nottingham Comparison of CO TPD from H 2 O, CH 3 OH and mixtures. 100 L CH 3 OH or H 2 O or a 1:1 mixture of a 1:1 mixture of CH 3 OH + H 2 O CH 3 OH + H 2 O deposited at 8 K. deposited at 8 K. 10 L CO deposited at 8 K. 8 K. Heating rate = 0.08 K s -1.

School of Chemistry University of Nottingham RAIR Spectroscopy of CH 3 OH 100 L CH 3 OH deposited at 8 K. deposited at 8 K. CH 3 OH film annealed at a given annealed at a given temperature for temperature for 30 mins. 30 mins. Sample cooled to 8 K before IR scan before IR scan recorded. recorded.

School of Chemistry University of Nottingham RAIR Spectroscopy of CO on H 2 O and CH 3 OH CO / H 2 O CO / CH 3 OH

School of Chemistry University of Nottingham TPD of CH 3 OH CH 3 OH deposited at 8 K. Heating rate = 0.08 K s -1 = 0.08 K s -1 Desorption profile fits zeroth order fits zeroth order desorption kinetics. desorption kinetics.

School of Chemistry University of Nottingham CO TPD: a probe of CH 3 OH morphology 100 L of CH 3 OH deposited at varied deposited at varied temperatures. temperatures. 10 L of CO deposited after sample cooled after sample cooled to 8 K. to 8 K. Heating rate = 0.08 K s -1 = 0.08 K s -1 As with H 2 O, the ability of CH 3 OH to ability of CH 3 OH to trap CO is reduced trap CO is reduced as the deposition as the deposition temperature temperature increases. increases.

School of Chemistry University of Nottingham Temperature Dependence of CO Trapping

School of Chemistry University of Nottingham TPD of the CO/(H 2 O + CH 3 OH) System 100 L (H 2 O + CH 3 OH) gas mixture deposited at 15 K. 5 L CO deposited at 15 K Heating rate = 0.08 K s -1

School of Chemistry University of Nottingham TPD of the CO/(H 2 O + CH 3 OH) System 100 L (H 2 O + CH 3 OH) gas mixture deposited at 15 K. 5 L CO deposited at 15 K Heating rate = 0.08 K s -1

School of Chemistry University of NottinghamConclusions There are strong similarities between the phase changes of H 2 O and CH 3 OH as probed by CO TPD experimentsThere are strong similarities between the phase changes of H 2 O and CH 3 OH as probed by CO TPD experiments Further investigation of desorption of the CO/(H 2 O + CH 3 OH system is required:Further investigation of desorption of the CO/(H 2 O + CH 3 OH system is required: –Desorption of co-deposited (CO + H 2 O + CH 3 OH) may prove more revealing of the phase transitions in H 2 O + CH 3 OH mixed ices. –Extension to IR spectroscopy of (H 2 O + CH 3 OH ) and (CO + H 2 O + CH 3 OH) will also help us to understand the system.

School of Chemistry University of NottinghamAcknowledgements 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.