School of ChemistrySchool of ChemistryUniversity of NottinghamUniversity of Nottingham

Probing the Morphology of Probing the Morphology of Interstellar Ice AnaloguesInterstellar Ice Analogues

In Memory of Rui Chen.In Memory of Rui Chen.

Mark CollingsMark CollingsSchool of Chemistry, University of NottinghamSchool of Chemistry, University of Nottingham

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TPD of CO/HTPD of CO/H22O SystemO System

CO desorption profile is strongly dependent on CO desorption profile is strongly dependent on the ice surface on which it was deposited.the ice surface on which it was deposited.

Trapping of CO only occurs when CO is Trapping of CO only occurs when CO is deposited on porous ASW, or as a mixture.deposited on porous ASW, or as a mixture.

30 30 g cmg cm-2-2 H H22O (~ 0.35 O (~ 0.35 m at 8 K)m at 8 K)0.07 0.07 g cmg cm-2-2 CO CO

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Summary of CO/HSummary of CO/H22O Thermal BehaviourO Thermal Behaviour

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Simulation of Desorption in the CO/HSimulation of Desorption in the CO/H22O O

SystemSystemWe developed a kinetic model of We developed a kinetic model of the thermal behaviour of the CO-the thermal behaviour of the CO-HH22O system.O system.– The model contains 16 reaction The model contains 16 reaction

steps.steps.– All but 3 of the kinetic parameters All but 3 of the kinetic parameters

were experimentally determined.were experimentally determined.– The remaining 3 were varied in the The remaining 3 were varied in the

model until the model matched the model until the model matched the experimental TPD results.experimental TPD results.

Collings Collings et alet al., 2003, ., 2003, Astron. & Space Sci.Astron. & Space Sci., , 285285, 633., 633.

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Simulation of Desorption in the CO/HSimulation of Desorption in the CO/H22O O

SystemSystemThe heating rate can was then The heating rate can was then slowed to values applicable slowed to values applicable during star formation.during star formation.

The model predicts the The model predicts the temperatures at which various temperatures at which various desorption processes occur in desorption processes occur in the interstellar medium.the interstellar medium.

Collings Collings et alet al., 2003, ., 2003, Astron. & Space Sci.Astron. & Space Sci., , 285285, 633., 633.

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Desorption of Other SpeciesDesorption of Other Species

A rapid survey of the behaviour of other A rapid survey of the behaviour of other species desorbing from Hspecies desorbing from H22O reveals O reveals three categories.three categories.

Water like: desorb coincidently with water.Water like: desorb coincidently with water.

CO-like: diffuse throughout the porous CO-like: diffuse throughout the porous structure of water ice at low structure of water ice at low temperature, showing two desorptions of temperature, showing two desorptions of trapped molecules, desorption from the trapped molecules, desorption from the HH22O surface, and (if deposited as a O surface, and (if deposited as a layer) desorption of the solid species.layer) desorption of the solid species.

Intermediate: show two desorptions of Intermediate: show two desorptions of trapped molecules, but a limited ability trapped molecules, but a limited ability to diffuse within porous Hto diffuse within porous H22O.O.

Collings Collings et alet al., 2004, ., 2004, Mon. Not. Roy. Astron. Soc.Mon. Not. Roy. Astron. Soc., , 354354, , 1133.1133.

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Comparison of CO TPD from HComparison of CO TPD from H22O, O,

CHCH33OH and mixtures.OH and mixtures.

100 L CH100 L CH33OH or HOH or H22O orO or a 1:1 mixture ofa 1:1 mixture of CHCH33OH + HOH + H22OO deposited at 8 K.deposited at 8 K.

10 L CO deposited at 10 L CO deposited at 8 K.8 K.

Heating rate = 0.08 K sHeating rate = 0.08 K s-1-1..

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RAIR Spectroscopy of CHRAIR Spectroscopy of CH33OHOH

100 L CH100 L CH33OHOH deposited at 8 K.deposited at 8 K.

CHCH33OH filmOH film annealed at a givenannealed at a given temperature fortemperature for 30 mins.30 mins.

Sample cooled to 8 KSample cooled to 8 K before IR scanbefore IR scan recorded.recorded.

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RAIR Spectroscopy of CO onRAIR Spectroscopy of CO onHH22O and CHO and CH33OHOH

CO / H2O CO / CH3OH

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TPD of CHTPD of CH33OHOH

CHCH33OH deposited at OH deposited at 8 K.8 K.

Heating rate Heating rate = 0.08 K s= 0.08 K s-1-1

Desorption profileDesorption profile fits zeroth orderfits zeroth order desorption kinetics.desorption kinetics.

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CO TPD: a probe of CHCO TPD: a probe of CH33OH morphologyOH morphology

100 L of CH100 L of CH33OHOH deposited at varieddeposited at varied temperatures.temperatures.

10 L of CO deposited10 L of CO deposited after sample cooledafter sample cooled to 8 K.to 8 K.

Heating rate Heating rate = 0.08 K s= 0.08 K s-1-1

As with HAs with H22O, theO, the ability of CHability of CH33OH toOH to trap CO is reducedtrap CO is reduced as the depositionas the deposition temperaturetemperature increases.increases.

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Temperature Dependence of CO TrappingTemperature Dependence of CO Trapping

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TPD of the CO/(HTPD of the CO/(H22O + CHO + CH33OH) SystemOH) System100 L (H100 L (H22O + CHO + CH33OH) gas mixture deposited at 15 K.OH) gas mixture deposited at 15 K.5 L CO deposited at 15 K5 L CO deposited at 15 KHeating rate = 0.08 K sHeating rate = 0.08 K s-1-1

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TPD of the CO/(HTPD of the CO/(H22O + CHO + CH33OH) SystemOH) System100 L (H100 L (H22O + CHO + CH33OH) gas mixture deposited at 15 K.OH) gas mixture deposited at 15 K.5 L CO deposited at 15 K5 L CO deposited at 15 KHeating rate = 0.08 K sHeating rate = 0.08 K s-1-1

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ConclusionsConclusions

• There are strong similarities between the phase changes of HThere are strong similarities between the phase changes of H22O O

and CHand CH33OH as probed by CO TPD experimentsOH as probed by CO TPD experiments

• Further investigation of desorption of the CO/(HFurther investigation of desorption of the CO/(H22O + CHO + CH33OH OH

system is required: system is required: – Desorption of co-deposited (CO + HDesorption of co-deposited (CO + H22O + CHO + CH33OH) may prove more OH) may prove more

revealing of the phase transitions in Hrevealing of the phase transitions in H22O + CHO + CH33OH mixed ices.OH mixed ices.

– Extension to IR spectroscopy of (HExtension to IR spectroscopy of (H22O + CHO + CH33OH ) and (CO + HOH ) and (CO + H22O + O +

CHCH33OH) will also help us to understand the system.OH) will also help us to understand the system.

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AcknowledgementsAcknowledgements

Dr Martin McCoustraDr Martin McCoustra

Rui Chen, Simon Green, Laura Tye, John ThrowerRui Chen, Simon Green, Laura Tye, John Thrower

The Leverhulme Trust, The University of Nottingham,The Leverhulme Trust, The University of Nottingham,

££££££ Engineering and Physical Sciences Research Council £Engineering and Physical Sciences Research Council £

££££ (EPSRC) U.K., ORS Awards.(EPSRC) U.K., ORS Awards.