School of Chemistry University of Nottingham The Morphology of Water Ice in Interstellar Ice Analogues CMD - CMMP Brighton, 8th April, 2002 School of Chemistry University of Nottingham Mark Collings John Dever, Mark Anderson, Helen Fraser, Martin McCoustra David A. Williams

School of Chemistry University of NottinghamOutline Background - ice on interstellar dust grains.Background - ice on interstellar dust grains. Temperature programmed desorption analysis of CO adsorbed on amorphous water ice.Temperature programmed desorption analysis of CO adsorbed on amorphous water ice. Reflection-absorption infrared spectroscopy of CO adsorbed on amorphous water ice.Reflection-absorption infrared spectroscopy of CO adsorbed on amorphous water ice. Kinetic simulation of CO desorption - a model of the behaviour of icy mantles during thermal processing.Kinetic simulation of CO desorption - a model of the behaviour of icy mantles during thermal processing. Conclusions; limitations of the model and the direction of future research.Conclusions; limitations of the model and the direction of future research.

School of Chemistry University of Nottingham Icy Mantles on Dust Grains Dust grains have a core of silicate or carbonaceous material.Dust grains have a core of silicate or carbonaceous material. –Dimensions typically in the sub-micron range.

School of Chemistry University of Nottingham Icy Mantles on Dust Grains Dust grains have a core of silicate or carbonaceous material.Dust grains have a core of silicate or carbonaceous material. A layer of hydrogenated (“polar”) ice grows around the core.A layer of hydrogenated (“polar”) ice grows around the core. –Dominated by H 2 O, with lower concentrations of CH 3 OH, NH 3, CH 4 etc. –Ice is formed by reaction of adsorbed atomic H, O, C, N. –Dehydrogenated species such as CO, CO 2, O 2 and N 2 may also be adsorbed in low concentrations. Silicate or Carbonaceous Core H2OH2O CH 3 OH NH 3 CH 4 H2OH2OH2OH2O

School of Chemistry University of Nottingham Icy Mantles on Dust Grains Dust grains have a core of silicate or carbonaceous material.Dust grains have a core of silicate or carbonaceous material. A layer of hydrogenated (“polar”) ice grows around the core.A layer of hydrogenated (“polar”) ice grows around the core. Dehydrogenated (“apolar”) ice is accreted in an outer layer.Dehydrogenated (“apolar”) ice is accreted in an outer layer. –Such ice contains CO, O 2, N 2, CO 2 etc. –The total thickness of ice may be up to 0.5  m. Silicate or Carbonaceous Core Hydrogenated Ice CO CO 2 O2O2 N2N2

School of Chemistry University of Nottingham Icy Mantles on Dust Grains In this model of ice accretion, dust grains are often described as having an “onion like” layered structure.In this model of ice accretion, dust grains are often described as having an “onion like” layered structure. Amorphous water ice is adsorbed with high density, highly porous structure. A phase change to less dense, lower porosity amorphous structure at 30 ~ 80 K has been identified.Amorphous water ice is adsorbed with high density, highly porous structure. A phase change to less dense, lower porosity amorphous structure at 30 ~ 80 K has been identified. – I hda I lda – I hda  I lda P. Jenniskens, D.F. Blake, Science, 265 (1994), 753. P. Jenniskens, D.F. Blake, Science, 265 (1994), 753. Silicate or Carbonaceous Core Hydrogenated Ice Dehydrogenated Ice

School of Chemistry University of Nottingham Temperature Programmed Desorption - CO adsorbed on amorphous water ice CO / H 2 O : a “simplistic” model of an ‘onion layered’ ice mantle with non-hydrogenated / hydrogenated layers.CO / H 2 O : a “simplistic” model of an ‘onion layered’ ice mantle with non-hydrogenated / hydrogenated layers. A layer of water ice is grown at 8 K - 17  g cm -2 of H 2 O : ~ 0.2  m.A layer of water ice is grown at 8 K - 17  g cm -2 of H 2 O : ~ 0.2  m. A layer of CO adsorbed over H 2 O at 8 K.A layer of CO adsorbed over H 2 O at 8 K. –Exposure is varied; the largest corresponds to 0.4  g cm -2

School of Chemistry University of Nottingham Temperature Programmed Desorption - varied CO exposure Multilayer CO desorption at ~ 25 K.Multilayer CO desorption at ~ 25 K. CO desorbs from water surface in the 30 ~ 70 K range. Evidence of multiple adsorption sites.CO desorbs from water surface in the 30 ~ 70 K range. Evidence of multiple adsorption sites. ‘Molecular volcano’ desorption at 140 K.‘Molecular volcano’ desorption at 140 K. Remaining CO desorption at 160 K is coincident with H 2 O desorption.Remaining CO desorption at 160 K is coincident with H 2 O desorption.

School of Chemistry University of Nottingham Temperature Programmed Desorption - varied H 2 O thickness H 2 O adsorption at 8 K with variable exposure.H 2 O adsorption at 8 K with variable exposure. –Largest corresponds to 28  g cm -2 of H 2 O : ~ 0.3  m. CO overlayer adsorbed at 8 KCO overlayer adsorbed at 8 K –0.07  g cm -2. Degree of CO entrapment increases with increasing H 2 O film thickness.Degree of CO entrapment increases with increasing H 2 O film thickness.

School of Chemistry University of Nottingham Temperature Programmed Desorption - varied temperature of water ice adsorption 57  g cm -2 of H 2 O (~ 0.6  m), and 0.07  g cm -2 CO.57  g cm -2 of H 2 O (~ 0.6  m), and 0.07  g cm -2 CO. In the lower (red) trace, H 2 O is grown over a pre-adsorbed CO overlayer at 8 K.In the lower (red) trace, H 2 O is grown over a pre-adsorbed CO overlayer at 8 K. In the upper (black) traces, H 2 O is grown at varying temperature, with a CO overlayer adsorbed onto the H 2 O film at 8 K.In the upper (black) traces, H 2 O is grown at varying temperature, with a CO overlayer adsorbed onto the H 2 O film at 8 K. The extent of CO entrapment is reduced with increasing H 2 O adsorption temperature.The extent of CO entrapment is reduced with increasing H 2 O adsorption temperature.

School of Chemistry University of Nottingham RAIRS - co-adsorbed CO and H 2 O RAIRS - co-adsorbed CO and H 2 O 60  g cm -2 of a ~ 5% CO in H 2 O gas mixture adsorbed at 8 K (~ 0.6  m).60  g cm -2 of a ~ 5% CO in H 2 O gas mixture adsorbed at 8 K (~ 0.6  m). Sample annealed for 5 minutes at the temperature indicated.Sample annealed for 5 minutes at the temperature indicated.

School of Chemistry University of Nottingham RAIRS - co-adsorbed CO and H 2 O RAIRS - co-adsorbed CO and H 2 O Two peak profile is typical of CO in a H 2 O matrix - two bonding sites.Two peak profile is typical of CO in a H 2 O matrix - two bonding sites. The exact nature of the larger feature at 2138 cm -1 remains controversial.The exact nature of the larger feature at 2138 cm -1 remains controversial. The smaller feature at 2152 cm -1 is attributed to CO bound to OH dangling bonds at the H 2 O surface.The smaller feature at 2152 cm -1 is attributed to CO bound to OH dangling bonds at the H 2 O surface. The loss of the 2152 cm -1 feature over the K range during I hda I lda phase change.The loss of the 2152 cm -1 feature over the K range during I hda  I lda phase change. CO retained to above 130 K.CO retained to above 130 K.

School of Chemistry University of Nottingham RAIRS - CO adsorbed on I hda RAIRS - CO adsorbed on I hda 57  g cm -2 of H 2 O adsorbed at 8 K (~ 0.6  m).57  g cm -2 of H 2 O adsorbed at 8 K (~ 0.6  m)  g cm -2 of CO adsorbed at 8 K.0.35  g cm -2 of CO adsorbed at 8 K. Sample annealed for 5 minutes at the temperature indicated.Sample annealed for 5 minutes at the temperature indicated. Two sharp features at 2143 and 2138 cm -1 are, respectively, the longitudinal and orthogonal C-O str modes of solid CO.Two sharp features at 2143 and 2138 cm -1 are, respectively, the longitudinal and orthogonal C-O str modes of solid CO. LST splitting is observed in RAIRS, and in transmission IR when a p-polarised IR source is used.LST splitting is observed in RAIRS, and in transmission IR when a p-polarised IR source is used.

School of Chemistry University of Nottingham RAIRS - CO adsorbed on I hda RAIRS - CO adsorbed on I hda Annealing to 15 K causes loss of multilayer CO peaks and growth of the 2152 cm -1 feature - diffusion of CO into the porous structure of H 2 O.Annealing to 15 K causes loss of multilayer CO peaks and growth of the 2152 cm -1 feature - diffusion of CO into the porous structure of H 2 O. Annealing to 20 K results in the same two peak profile observed for the mixture.Annealing to 20 K results in the same two peak profile observed for the mixture. Annealing above 45 K causes loss of the 2152 cm -1 feature, as occurs for the mixture.Annealing above 45 K causes loss of the 2152 cm -1 feature, as occurs for the mixture. A feature due to trapped CO remains after annealing at up to 130 K.A feature due to trapped CO remains after annealing at up to 130 K.

School of Chemistry University of Nottingham RAIRS - CO adsorbed on I lda RAIRS - CO adsorbed on I lda 57  g cm -2 of H 2 O adsorbed at 80 K. Amorphous water ice in the I lda phase formed.57  g cm -2 of H 2 O adsorbed at 80 K. Amorphous water ice in the I lda phase formed  g cm -2 of CO adsorbed at 8 K.0.35  g cm -2 of CO adsorbed at 8 K. Sample annealed for 5 minutes at the temperature indicated.Sample annealed for 5 minutes at the temperature indicated.

School of Chemistry University of Nottingham RAIRS - CO adsorbed on I lda RAIRS - CO adsorbed on I lda Upon annealing, relatively little growth of the 2152 cm -1 feature.Upon annealing, relatively little growth of the 2152 cm -1 feature. Complete loss of CO features with annealing to 50 K.Complete loss of CO features with annealing to 50 K. Since the phase change is ‘complete’ at the time of H 2 O adsorption, no mechanism remains to trap subsequently adsorbed CO.Since the phase change is ‘complete’ at the time of H 2 O adsorption, no mechanism remains to trap subsequently adsorbed CO.

School of Chemistry University of Nottingham Summary of Phase Change Behaviour

School of Chemistry University of Nottingham Summary of Phase Change Behaviour Adsorption of I hda and CO overlayer.Adsorption of I hda and CO overlayer.

School of Chemistry University of Nottingham Summary of Phase Change Behaviour Adsorption of I hda and CO overlayer.Adsorption of I hda and CO overlayer. Diffusion of CO into pores; desorption of solid CO.Diffusion of CO into pores; desorption of solid CO.

School of Chemistry University of Nottingham Summary of Phase Change Behaviour Adsorption of I hda and CO overlayer.Adsorption of I hda and CO overlayer. Diffusion of CO into pores; desorption of solid CO.Diffusion of CO into pores; desorption of solid CO.

School of Chemistry University of Nottingham Summary of Phase Change Behaviour Adsorption of I hda and CO overlayer.Adsorption of I hda and CO overlayer. Diffusion of CO into pores; desorption of solid CO.Diffusion of CO into pores; desorption of solid CO. Phase Change I hda I lda Desorption of CO from external and pore surfaces.Phase Change I hda  I lda Desorption of CO from external and pore surfaces.

School of Chemistry University of Nottingham Summary of Phase Change Behaviour Adsorption of I hda and CO overlayer.Adsorption of I hda and CO overlayer. Diffusion of CO into pores; desorption of solid CO.Diffusion of CO into pores; desorption of solid CO. Phase Change I hda I lda Desorption of CO from external and pore surfaces.Phase Change I hda  I lda Desorption of CO from external and pore surfaces.

School of Chemistry University of Nottingham Summary of Phase Change Behaviour Adsorption of I hda and CO overlayer.Adsorption of I hda and CO overlayer. Diffusion of CO into pores; desorption of solid CO.Diffusion of CO into pores; desorption of solid CO. Phase Change I hda I lda Desorption of CO from external and pore surfaces.Phase Change I hda  I lda Desorption of CO from external and pore surfaces. Crystallisation of H 2 O - CO released abruptly as new pathways to surface form.Crystallisation of H 2 O - CO released abruptly as new pathways to surface form.

School of Chemistry University of Nottingham Summary of Phase Change Behaviour Adsorption of I hda and CO overlayer.Adsorption of I hda and CO overlayer. Diffusion of CO into pores; desorption of solid CO.Diffusion of CO into pores; desorption of solid CO. Phase Change I hda I lda Desorption of CO from external and pore surfaces.Phase Change I hda  I lda Desorption of CO from external and pore surfaces. Crystallisation of H 2 O - CO released abruptly as new pathways to surface form.Crystallisation of H 2 O - CO released abruptly as new pathways to surface form. CO released as H 2 O desorbs.CO released as H 2 O desorbs.

School of Chemistry University of Nottingham Kinetic Simulations CO(s) CO(g) sublimation CO(i) CO(g) desorptionCO(s)  CO(g) sublimation CO(i)  CO(g) desorption CO(s) CO(i-p) diffusion into poresCO(s)  CO(i-p) diffusion into pores CO(i-p) CO(g-p) desorption in pores CO(g-p) CO(g) diffusion out of pores CO(g-p) CO(i-p) re-adsorption in poresCO(i-p)  CO(g-p) desorption in pores CO(g-p)  CO(g) diffusion out of pores CO(g-p)  CO(i-p) re-adsorption in pores CO(i-p) CO(t-p) trappingCO(i-p)  CO(t-p) trapping CO(t-p) CO(g) releaseCO(t-p)  CO(g) release Steps in the Kinetic Model

School of Chemistry University of Nottingham Kinetic Simulations Steps in the Kinetic Model Steps in the Kinetic Model CO(s) CO(g) sublimation CO(i) CO(g) desorptionCO(s)  CO(g) sublimation CO(i)  CO(g) desorption CO(s) CO(i-p) diffusion into poresCO(s)  CO(i-p) diffusion into pores CO(i-p) CO(g-p) desorption in pores CO(g-p) CO(g) diffusion out of pores CO(g-p) CO(i-p) re-adsorption in poresCO(i-p)  CO(g-p) desorption in pores CO(g-p)  CO(g) diffusion out of pores CO(g-p)  CO(i-p) re-adsorption in pores CO(i-p) CO(t-p) trappingCO(i-p)  CO(t-p) trapping CO(t-p) CO(g) releaseCO(t-p)  CO(g) release

School of Chemistry University of Nottingham Conclusions and Implications CO can diffuse into the porous structure of amorphous H 2 O at temperatures of below 20 K.CO can diffuse into the porous structure of amorphous H 2 O at temperatures of below 20 K. The phase transition I hda I lda can trap CO, causing it to be retained until desorption of the amorphous H 2 O.The phase transition I hda  I lda can trap CO, causing it to be retained until desorption of the amorphous H 2 O. The structure of the amorphous water may greatly influence the desorption of icy mantles in the interstellar environment;The structure of the amorphous water may greatly influence the desorption of icy mantles in the interstellar environment; –impacts on collapse of star forming regions,collapse of star forming regions, temperature dependent gas phase chemistry andtemperature dependent gas phase chemistry and temperature dependent solid phase chemistry.temperature dependent solid phase chemistry.

School of Chemistry University of Nottingham Limitations of the Model - Future Directions How do other adsorbates behave?How do other adsorbates behave? Effect of other molecules?Effect of other molecules? Effect of substrate?Effect of substrate? Effect of UV irradiation and cosmic ray bombardment?Effect of UV irradiation and cosmic ray bombardment? Does reactively formed ice have a similar structure to accreted ice?Does reactively formed ice have a similar structure to accreted ice?

School of Chemistry University of NottinghamAcknowledgements University of Nottingham, School of ChemistryUniversity of Nottingham, School of Chemistry –John Dever –Mark Anderson –Dr Martin McCoustra Leiden University, Raymond and Beverly Sackler Laboratory for AstrophysicsLeiden University, Raymond and Beverly Sackler Laboratory for Astrophysics –Dr Helen Fraser University College London, Department of Physics and AstronomyUniversity College London, Department of Physics and Astronomy –Prof David A. Williams –Dr Serena Viti £ £ £ £ £ PPARC, EPSRC, Nottingham University £ £ £ £ ££ £ £ £ £ PPARC, EPSRC, Nottingham University £ £ £ £ £