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Laboratory Infrared Studies of Interstellar Ices. Mark Collings School of Chemistry University of Nottingham. Astrochemistry From Laboratory to Telescope Cardiff - 6 th January 2005. Observations of Interstellar Ices. Figure reproduced from Whittet et.al. 1996, A&A , 315 , L357.
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Laboratory Infrared Studies of Interstellar Ices Mark Collings School of Chemistry University of Nottingham Astrochemistry From Laboratory to Telescope Cardiff - 6th January 2005.
Observations of Interstellar Ices Figure reproduced from Whittet et.al. 1996, A&A, 315, L357.
RAIR Spectra of CO / H2O Mixture • CO and H2O are co-deposited at 8 K, to give a film of ~ 5 % CO. • Spectra are recorded in reflection-absorption infrared configuration. • The film is annealed then cooled to the base temperature (8 K) before the IR scan is recorded. Therefore all observed changes are irreversible.
Ballistic CO Deposition • H2O film deposited at varying temperature. • CO deposited at 8 K – ballistic adsorption; i.e. “stick and stop”. • Roughly half the “surface” covered in each case. • Adsorbed CO samples surface sites in a statistical ratio.
CO Deposition at Elevated Temperature • CO adsorbed at 30 K, at which temperature CO molecules are able to diffuse across the water ice surface. • CO adsorbed at the strongest adsorption sites first.
CO Saturation of Varying Ice Surfaces • CO deposition at 30 K continued until the surface is saturated. • Ratio of peaks changes for CO adsorption on porous ices. • High frequency peak shifts to lower wavenumber
Difference Spectra – CO/H2O Mixture • Spectra from the set shown previously.
Difference Spectra – CO/H2O Mixture • Spectra from the set shown previously. • Difference spectrum highlights the changes.
Difference Spectra – Saturated CO Adsorption • Difference spectrum between saturated monolayer and sub-monolayer (i.e. unsaturated monolayer).
Difference Spectra – Multilayer • Difference spectrum between multilayer and sub-monolayer where CO adsorption is ballistic.
Difference Spectra – Multilayer on Crystalline Ice Ic • Water ice deposited at 140 K to give a cubic crystalline structure. • The ice surface is as ordered as we can make it.
The CO Stretch Absorption in Observational Spectra Figure reproduced from Pontoppidan et.al. 2003, A&A, 408, 981.
Evolution of Layered CO-H2O Ice Figure reproduced from Fraser et.al. 2004, MNRAS, 353, 59.
RAIR Spectrum of Pure CO • CO stretch observed at 2139 cm-1 in astronomical spectra and laboratory transmission experiments - transverse optical (TO) mode. • CO stretch observed at 2142 cm-1 in RAIRS experiments – longitudinal optical (LO) mode.
Conclusions • IR spectroscopy of CO is an ideal probe of the structure of water ice, an important consideration when studying chemistry occurring within water films. • Laboratory experiments indicate that water ice on interstellar dust grains is porous, but that the sites that give rise to the 2152 cm-1 CO stretch feature are blocked. • The optical properties of an underlying film can influence the position and size of observed bands in an overlayer of CO.
Acknowledgements • Martin McCoustra, John Dever University of Nottingham • Helen Fraser University of Strathclyde • Elisabetta Palumbo, Giuseppe Baratta Catania Astrophysical Observatory • Funding by &