The Morphology of Water Ice in Interstellar Ice Analogues

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

Outline • Background - ice on interstellar dust grains. • Temperature programmed desorption analysis 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. • Conclusions; limitations of the model and the direction of future research.

Icy Mantles on Dust Grains • Dust grains have a core of silicate or carbonaceous material. • Dimensions typically in the sub-micron range.

Icy Mantles on Dust Grains • Dust grains have a core of silicate or carbonaceous material. • A layer of hydrogenated (“polar”) ice grows around the core. • Dominated by H2O, with lower concentrations of CH3OH, NH3, CH4 etc. • Ice is formed by reaction of adsorbed atomic H, O, C, N. • Dehydrogenated species such as CO, CO2, O2 and N2 may also be adsorbed in low concentrations. CH3OH H2O H2O Silicate or Carbonaceous Core CH4 NH3 H2O

Icy Mantles on Dust Grains • Dust grains have a core of silicate or carbonaceous material. • A layer of hydrogenated (“polar”) ice grows around the core. • Dehydrogenated (“apolar”) ice is accreted in an outer layer. • Such ice contains CO, O2, N2, CO2 etc. • The total thickness of ice may be up to 0.5 mm. CO O2 Silicate or Carbonaceous Core N2 Hydrogenated Ice CO2

Icy Mantles on Dust Grains • 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. • Ihda  Ilda P. Jenniskens, D.F. Blake, Science, 265 (1994), 753. Silicate or Carbonaceous Core Hydrogenated Ice Dehydrogenated Ice

Temperature Programmed Desorption- CO adsorbed on amorphous water ice • CO / H2O : 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 mg cm-2 of H2O : ~ 0.2 mm. • A layer of CO adsorbed over H2O at 8 K. • Exposure is varied; the largest corresponds to 0.4 mg cm-2

Temperature Programmed Desorption- varied CO exposure • Multilayer CO desorption at ~ 25 K. • CO desorbs from water surface in the 30 ~ 70 K range. Evidence of multiple adsorption sites. • ‘Molecular volcano’ desorption at 140 K. • Remaining CO desorption at 160 K is coincident with H2O desorption.

Temperature Programmed Desorption- varied H2O thickness • H2O adsorption at 8 K with variable exposure. • Largest corresponds to 28 mg cm-2 of H2O : ~ 0.3 mm. • CO overlayer adsorbed at 8 K • 0.07 mg cm-2. • Degree of CO entrapment increases with increasing H2O film thickness.

Temperature Programmed Desorption- varied temperature of water ice adsorption • 57 mg cm-2 of H2O (~ 0.6 mm), and 0.07 mg cm-2 CO. • In the lower (red) trace, H2O is grown over a pre-adsorbed CO overlayer at 8 K. • In the upper (black) traces, H2O is grown at varying temperature, with a CO overlayer adsorbed onto the H2O film at 8 K. • The extent of CO entrapment is reduced with increasing H2O adsorption temperature.

RAIRS- co-adsorbed CO and H2O • 60 mg cm-2 of a ~ 5% CO in H2O gas mixture adsorbed at 8 K (~ 0.6 mm). • Sample annealed for 5 minutes at the temperature indicated.

RAIRS- co-adsorbed CO and H2O • Two peak profile is typical of CO in a H2O matrix - two bonding sites. • 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 H2O surface. • The loss of the 2152 cm-1 feature over the 30 - 70 K range during Ihda Ilda phase change. • CO retained to above 130 K.

RAIRS- CO adsorbed on Ihda • 57 mg cm-2 of H2O adsorbed at 8 K (~ 0.6 mm). • 0.35 mg cm-2 of CO adsorbed at 8 K. • 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-Ostr modes of solid CO. • LST splitting is observed in RAIRS, and in transmission IR when a p-polarised IR source is used.

RAIRS- CO adsorbed on Ihda • 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 H2O. • 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. • A feature due to trapped CO remains after annealing at up to 130 K.

RAIRS- CO adsorbed on Ilda • 57 mg cm-2 of H2O adsorbed at 80 K. Amorphous water ice in the Ilda phase formed. • 0.35 mg cm-2 of CO adsorbed at 8 K. • Sample annealed for 5 minutes at the temperature indicated.

RAIRS- CO adsorbed on Ilda • Upon annealing, relatively little growth of the 2152 cm-1 feature. • Complete loss of CO features with annealing to 50 K. • Since the phase change is ‘complete’ at the time of H2O adsorption, no mechanism remains to trap subsequently adsorbed CO.

Summary of Phase Change Behaviour

Summary of Phase Change Behaviour • Adsorption of Ihda and CO overlayer.

Summary of Phase Change Behaviour • Adsorption of Ihda and CO overlayer. • Diffusion of CO into pores; desorption of solid CO.

Summary of Phase Change Behaviour • Adsorption of Ihda and CO overlayer. • Diffusion of CO into pores; desorption of solid CO. • Phase Change Ihda Ilda Desorption of CO from external and pore surfaces.

Summary of Phase Change Behaviour • Adsorption of Ihda and CO overlayer. • Diffusion of CO into pores; desorption of solid CO. • Phase Change Ihda Ilda Desorption of CO from external and pore surfaces. • Crystallisation of H2O - CO released abruptly as new pathways to surface form.

Summary of Phase Change Behaviour • Adsorption of Ihda and CO overlayer. • Diffusion of CO into pores; desorption of solid CO. • Phase Change Ihda Ilda Desorption of CO from external and pore surfaces. • Crystallisation of H2O - CO released abruptly as new pathways to surface form. • CO released as H2O desorbs.

Kinetic Simulations Steps in the Kinetic Model • CO(s)  CO(g) sublimation CO(i)  CO(g) desorption • CO(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 pores • CO(i-p)  CO(t-p) trapping • CO(t-p)  CO(g) release

Kinetic Simulations • Steps in the Kinetic Model • CO(s)  CO(g) sublimation CO(i)  CO(g) desorption • CO(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 pores • CO(i-p)  CO(t-p) trapping • CO(t-p)  CO(g) release

Conclusions and Implications • CO can diffuse into the porous structure of amorphous H2O at temperatures of below 20 K. • The phase transition Ihda Ilda can trap CO, causing it to be retained until desorption of the amorphous H2O. • 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, • temperature dependent gas phase chemistry and • temperature dependent solid phase chemistry.

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

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

The Morphology of Water Ice in Interstellar Ice Analogues