Laboratory Infrared Studies of Interstellar Ices

School of ChemistrySchool of ChemistryUniversity of NottinghamUniversity of Nottingham

Laboratory Infrared Studies Laboratory Infrared Studies of Interstellar Icesof Interstellar Ices

Mark CollingsMark Collings

School of ChemistrySchool of Chemistry

University of NottinghamUniversity of Nottingham

Astrochemistry Astrochemistry From Laboratory to TelescopeFrom Laboratory to Telescope

Cardiff - 6Cardiff - 6thth January 2005. January 2005.


Observations of Interstellar IcesObservations of Interstellar Ices

Figure reproduced from Whittet et.al. 1996, A&A, 315, L357.


Ice Mantles on Interstellar Dust Ice Mantles on Interstellar Dust GrainsGrains


RAIR Spectra of CO / HRAIR Spectra of CO / H22O MixtureO Mixture

• CO and HCO and H22O are co-deposited O are co-deposited

at 8 K, to give a film of ~ 5 % at 8 K, to give a film of ~ 5 % CO.CO.

• Spectra are recorded in Spectra are recorded in reflection-absorption infrared reflection-absorption infrared configuration.configuration.

• The film is annealed then The film is annealed then cooled to the base cooled to the base temperature (8 K) before the temperature (8 K) before the IR scan is recorded. Therefore IR scan is recorded. Therefore all observed changes are all observed changes are irreversibleirreversible..


Ballistic CO DepositionBallistic CO Deposition

• HH22O film deposited at varying O film deposited at varying

temperature.temperature.

• CO deposited at 8 K – CO deposited at 8 K – ballistic adsorption; ballistic adsorption; i.e.i.e. “stick “stick and stop”.and stop”.

• Roughly half the “surface” Roughly half the “surface” covered in each case. covered in each case.

• Adsorbed CO samples surface Adsorbed CO samples surface sites in a statistical ratio.sites in a statistical ratio.


CO Deposition at Elevated TemperatureCO Deposition at Elevated Temperature

• CO adsorbed at 30 K, at CO adsorbed at 30 K, at which temperature CO which temperature CO molecules are able to diffuse molecules are able to diffuse across the water ice surface.across the water ice surface.

• CO adsorbed at the strongest CO adsorbed at the strongest adsorption sites first.adsorption sites first.


CO Saturation of Varying Ice SurfacesCO Saturation of Varying Ice Surfaces

• CO deposition at 30 K CO deposition at 30 K continued until the surface is continued until the surface is saturated.saturated.

• Ratio of peaks changes for Ratio of peaks changes for CO adsorption on porous ices.CO adsorption on porous ices.

• High frequency peak shifts to High frequency peak shifts to lower wavenumberlower wavenumber


Difference Spectra – CO/HDifference Spectra – CO/H22O MixtureO Mixture

• Spectra from the set shown Spectra from the set shown previously.previously.


Difference Spectra – CO/HDifference Spectra – CO/H22O MixtureO Mixture

• Spectra from the set shown Spectra from the set shown previously.previously.

• Difference spectrum Difference spectrum highlights the changes.highlights the changes.


Difference Spectra – Saturated CO AdsorptionDifference Spectra – Saturated CO Adsorption

• Difference spectrum between Difference spectrum between saturated monolayer and sub-saturated monolayer and sub-monolayer (monolayer (i.e.i.e. unsaturated unsaturated monolayer).monolayer).


Difference Spectra – MultilayerDifference Spectra – Multilayer

• Difference spectrum between Difference spectrum between multilayer and sub-monolayer multilayer and sub-monolayer where CO adsorption is where CO adsorption is ballistic.ballistic.


Difference Spectra – Multilayer on Crystalline Ice IDifference Spectra – Multilayer on Crystalline Ice Icc

• Water ice deposited at 140 K Water ice deposited at 140 K to give a cubic crystalline to give a cubic crystalline structure.structure.

• The ice surface is as ordered The ice surface is as ordered as we can make it.as we can make it.


The CO Stretch Absorption in The CO Stretch Absorption in Observational SpectraObservational Spectra

Figure reproduced from Pontoppidan et.al. 2003, A&A, 408, 981.


Evolution of Layered CO-HEvolution of Layered CO-H22O IceO Ice

Figure reproduced from Fraser et.al. 2004, MNRAS, 353, 59.


RAIR Spectrum of Pure CORAIR Spectrum of Pure CO

• CO stretch observed CO stretch observed at 2139 cmat 2139 cm-1-1 in in astronomical spectra astronomical spectra and laboratory and laboratory transmission transmission experiments - experiments - transverse optical transverse optical (TO) mode.(TO) mode.

• CO stretch observed CO stretch observed at 2142 cmat 2142 cm-1-1 in in RAIRS experiments RAIRS experiments – longitudinal – longitudinal optical (LO) mode.optical (LO) mode.


NN22 Deposition Deposition






















CO Adsorption on NCO Adsorption on N22 Films Films


ConclusionsConclusions

• IR spectroscopy of CO is an ideal probe of the structure IR spectroscopy of CO is an ideal probe of the structure of water ice, an important consideration when studying of water ice, an important consideration when studying chemistry occurring within water films.chemistry occurring within water films.

• Laboratory experiments indicate that water ice on Laboratory experiments indicate that water ice on interstellar dust grains is porous, but that the sites that interstellar dust grains is porous, but that the sites that give rise to the 2152 cmgive rise to the 2152 cm-1-1 CO stretch feature are CO stretch feature are blocked.blocked.

• The optical properties of an underlying film can The optical properties of an underlying film can influence the position and size of observed bands in an influence the position and size of observed bands in an overlayer of CO.overlayer of CO.


AcknowledgementsAcknowledgements

• Martin McCoustra, John DeverMartin McCoustra, John DeverUniversity of NottinghamUniversity of Nottingham

• Helen FraserHelen FraserUniversity of StrathclydeUniversity of Strathclyde

• Elisabetta Palumbo, Giuseppe BarattaElisabetta Palumbo, Giuseppe BarattaCatania Astrophysical ObservatoryCatania Astrophysical Observatory

• Funding byFunding by

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Laboratory Infrared Studies of Interstellar Ices