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Kamel Gadallah and Sven Khoeler
Peeling an Icy Onion... What Recent Experiments May Tell Us About the Structure and Evolution of Icy Dust Grains Martin McCoustra Alexander Rosu-Finsen, Demian Marchione, Ali Abdulgalil, Victoria Frankland, John Thrower and Mark Collings Kamel Gadallah and Sven Khoeler
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The Icy Onion… Time Diffuse Environments Dense Environments
K 102 cm-3 Lots of Photons Dense Environments K 105 – 107 cm-3 Few Photons Bare Grain H2O-rich Layer CO-rich Layer Complex Organics
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Peeling the Onion… H2O TPD is always zero order on silica which suggests it clusters on the surface When does this process begin? Even at 18 K, we observe O-H stretch intensity increases even when no H2O is added to the surface! Agglomeration 3
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Peeling the Onion… Rate of agglomeration increases with temperature
Arrhenius analysis suggests… Barrier to H2O diffusion on amorphous silica is around 2 kJ mol-1 De-wetting of H2O from silica even at the lowest of temperatures on relatively short timescales (a few 100s of years) Activation barrier drops to zero above 25 K coincident with the start of the pore-collapse process in ballistically deposited porous amorphous solid water (p-ASW) Ea ≈ 2 kJ mol-1 Ea ≈ 0 kJ mol-1 T ≈ 25 K 4
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Peeling the Onion… Simple kinetic model based on these observations assuming a cooling environment… Time Temperature
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Peeling the Onion… H2O ice may not form a continuous layer either on silica or carbonaceous substrates. Rather we will see growth occurring in three dimensional islands on the substrate! Reaction enthalpy release accelerates agglomeration and promotes compaction (p-ASW c-ASW) and amorphisation (CSW c-ASW) Peeling the Astronomical Onion A. Rosu-Finsen, D. Marchione, T. L. Salter, J. Stubbing, W. A. Brown and M. R. S. McCoustra, Phys. Chem. Chem. Phys., 2016, 18, Impact of Oxygen Chemistry on Model Interstellar Grain Surfaces A. Rosu-Finsen and M. R. S. McCoustra, Phys. Chem. Chem. Phys., 2018, 20,
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Cooking the Onion… Icy grains are cooked by a combination of optical radiation from the interstellar radiation field and charged particle radiation The former is effectively attenuated as the density of the cloud increases so that in dense objects only the internal VUV is normally considered
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Cooking the Onion… Where adsorbates can absorb optical radiation, we can see efficient desorption of the adsorbate Direct Indirect
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Cooking the Onion… Where adsorbates can absorb optical radiation, we can see efficient desorption of the adsorbate Direct Indirect
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Cooking the Onion… In the indirect process, the presence of C6H6 promotes H2O desorption Cross-section for the process can be estimated from PSD curves 110-19 cm2 at 250 nm cf. 410-19 cm2 for C6H6 itself Suggests an efficiency approaching 0.25 cf. VUV PSD which has a modest efficiency of around 5×10-3 10
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Cooking the Onion… S/W/B H2O ice is a unique substrate when it comes to non-thermal interactions involving high-energy photons and charged particles Long-lived excitons generated by these radiations can promote efficient desorption of species weakly hydrogen bound to the ice surface (e.g. C6H6) Cross-section ca cm2
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Cooking the Onion… H2O EPD was investigated a combination of TPD and RAIRS (looking only at total loss and not what’s lost) and found to have a cross-section of ca. 510-18 cm2 Less efficient than C6H6 desorption due to strong hydrogen bonding!
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Cooking the Onion… Simple toy models including re-adsorption can be constructed to investigate the impact of thermal and non-thermal desorption These can be used to look at simple systems approaching equilibrium if we ignore thermal desorption Support the idea that non-thermal desorption might produce non-volatiles in low temperature environments But dominant promoter is CR-generated excitons not photons! 13
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Cooking the Onion… Simulations allow us to estimate the gas phase concentration of H2O in the core of a quiescent object like Barnard 68 Value calculated is some 103 times too large… but why? Efficient routes for destruction of H2O in the gas phase? CO Overlayer capping? Other ideas? 14
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Cooking the Onion… Electronic excitations in adsorbates and in H2O ice itself provide a pathway to promote physics and chemistry which is mediated by the hydrogen-bonding network in the latter. Exciton-promoted desorption driven by charged particle interactions is more important in dense environments than photon-induced desorption. Surface Science Investigations of Photoprocesses in Model Interstellar Ices J. D. Thrower, M. P. Collings, M. R. S. McCoustra, D. J. Burke, W. A. Brown, A. Dawes, P. D. Holtom, P. Kendall, N. J. Mason, F. Jamme, H. J. Fraser, I. P. Clark and A. W. Parker, J. Vac. Sci. Technol. A, 2008, 26, Efficient C6H6 Desorption from H2O Ices Induced by Low Energy Electrons D. Marchione, J. Thrower and M. R. S. McCoustra, Phys. Chem. Chem. Phys., 2016, 18,
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Adding Some CO to the Mix…
Lots of recent work from us and others looking at CO binding energies on different substrates… H2O (11 – 17 kJ mol-1), silica (6 – 10 kJ mol-1), graphite… Assuming only thermal processes, we would expect CO to favour binding to the H2O surface Introducing non-thermal desorption, especially H2O exciton–mediated processes suggests that segregation of CO from H2O in a low coverage and low temperature regime can occur
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Adding Some CO to the Mix…
CO might begin to accrete on H2O-free regions of grain surfaces simply because of differences in the efficiency of non-thermal desorption. This should be observable in appropriate environments! Probing Model Interstellar Grain Surfaces with Small Molecules M. P. Collings, V. L. Frankland, J. Lasne, D. Marchione, A. Rosu-Finsen, and M. R. S. McCoustra, Mon. Not. Roy. Astron. Soc., 2015, 449, Peeling the Astronomical Onion A. Rosu-Finsen, D. Marchione, T. L. Salter, J. Stubbing, W. A. Brown and M. R. S. McCoustra, Phys. Chem. Chem. Phys., 2016, 18,
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Some Additional Flavour…
Is H2O unique as a substrate? Comparison with CH3OH (red) and (CH3CH2)2O (blue) Linear hydrogen bonded chains and no hydrogen bonding Fast process seen in H2O switches off Though O centre in the organics may look like that in H2O there are more relaxation pathways open to excitons due to the presence of the C atoms
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Some Additional Flavour…
H2O CH3OH (CH3CH2)2O Care is required in these experiments to ensure we know what is desorbing We must look at the gas phase as well as the surface Not substrate desorption but H2 desorption!
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Some Additional Flavour…
Clearly illustrated with H2O where H2O is not observed rather we see H2, O2 and other species from dissociation in the exciton state at or near the surface
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and M. R. S. McCoustra, ACS Earth Space Chem., 2017, 1, 209-215
Cooking the Onion… Dehydrogenation of hydrogen-rich organics is likely to be a significant pathway associated with non-thermal processing. Electron-Promoted Desorption from Water Ice Surfaces: Neutral Gas-phase Products A. G. G. M. Abdulgalil, A. Rosu-Finsen, D. Marchione, J. D. Thrower, M. P. Collings and M. R. S. McCoustra, ACS Earth Space Chem., 2017, 1, Molecular Hydrogen Production from Amorphous Solid Water in Low Energy Electron Irradiation K. A. K. Gadallah, D. Marchione, S. P. K. Koehler and M. R. S. McCoustra, Phys. Chem. Chem. Phys., 2017, 19, Electron-induced Chemistry: Preliminary Comparative Studies of Hydrogen Production from Water, Methanol, and Diethyl Ether D. Marchione and M. R. S. McCoustra, ACS Earth Space Chem., 2017, 1,
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Conclusions H2O de-wets from silica and carbonaceous materials as it reactively accretes – Grains with “wet” and “dry” areas CR-induced secondary electron-promoted H2O desorption is more efficient than VUV photodesorption and will slow H2O ice accumulation in cooling environments CO accretion (and hence organic formation) on silica surfaces may be favoured compared to H2O surfaces even though binding energies suggest otherwise (6 – 10 versus kJ mol-1) as an efficient exciton-mediated desorption channel seen on H2O likely does not operate on silica! Accumulation of hydrogen-rich species is tempered by exciton-promoted dehydrogenation which also means that molecular non-thermal desorption is unlikely!
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Acknowledgements Dr.’s Mark Collings and Jerome Lasne
John Dever, Simon Green, Rui Chen, John Thrower, Vicky Frankland, Ali Abdulgalil, Demian Marchione, Alex Rosu-Finsen and Skandar Taj ££ Leverhulme Trust University of Nottingham Heriot-Watt University This work was supported by of the U.K. Science and Technology Facilities Council (STFC, ST/M001075/1), the U.K. Engineering and Physical Sciences Research Council (EPSRC, GR/T27044/02) and the European Community FP7-ITN Marie-Curie Programme (LASSIE Project, Grant Agreement ).
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