Laboratory Infrared Studies of Interstellar Ices

1. Laboratory Infrared Studies of Interstellar Ices Mark Collings School of Chemistry University of Nottingham Astrochemistry From Laboratory to Telescope Cardiff - 6th January 2005.

2. Observations of Interstellar Ices Figure reproduced from Whittet et.al. 1996, A&A, 315, L357.

3. Ice Mantles on Interstellar Dust Grains

4. 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.

5. 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.

6. 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.

7. 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

8. Difference Spectra – CO/H2O Mixture • Spectra from the set shown previously.

9. Difference Spectra – CO/H2O Mixture • Spectra from the set shown previously. • Difference spectrum highlights the changes.

10. Difference Spectra – Saturated CO Adsorption • Difference spectrum between saturated monolayer and sub-monolayer (i.e. unsaturated monolayer).

11. Difference Spectra – Multilayer • Difference spectrum between multilayer and sub-monolayer where CO adsorption is ballistic.

12. 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.

13. The CO Stretch Absorption in Observational Spectra Figure reproduced from Pontoppidan et.al. 2003, A&A, 408, 981.

14. Evolution of Layered CO-H2O Ice Figure reproduced from Fraser et.al. 2004, MNRAS, 353, 59.

15. 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.

16. N2 Deposition

17. N2 Deposition

18. N2 Deposition

19. N2 Deposition

20. N2 Deposition

21. N2 Deposition

22. N2 Deposition

23. N2 Deposition

24. N2 Deposition

25. N2 Deposition

26. N2 Deposition

27. CO Adsorption on N2 Films

28. 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.

29. Acknowledgements • Martin McCoustra, John Dever University of Nottingham • Helen Fraser University of Strathclyde • Elisabetta Palumbo, Giuseppe Baratta Catania Astrophysical Observatory • Funding by &