1. 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

2. Thanks…
S. Taylor and D. A. Williams, Chemistry in Britain, 1993,

3. The Icy Onion… Time  Diffuse Environments Dense EnvironmentsK
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

4. 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 consistent with agglomeration!
Rate of agglomeration increases temperature! 4

5. Peeling the Onion… 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 5

6. Peeling the Onion…
Simple kinetic model based on these observations assuming a cooling environment…
Time 
Temperature 

7. Peeling the Onion…
H2O ice is unlikely to 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., Submitted

8. Adding Some CO to the Mix…
Lots of recent work from us and others looking at CO binding energies on different substrates… H2O, silica, graphite…
Simple kinetic simulations show that variation of binding energies can lead to segregation of CO from H2O in a low coverage and low temperature regime

9. Adding Some CO to the Mix…
CO might begin to accrete on H2O-free regions of grain surfaces simply because of binding energy difference and easy diffusion.
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,

10. 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 object only the internal VUV is normally considered

11. Cooking the Onion…
Where adsorbates can absorb optical radiation, we can see efficient desorption of the adsorbate
Direct
Indirect

12. Cooking the Onion…
To illustrate the indirect process, the presence of C6H6 promotes H2O desorption
Cross-section for the process can be estimated from PSD curves 110-19 cm2 at 250 nm cf. 410-19 cm2 for C6H6 itself
Suggests an efficiency approaching 0.25 cf. VUV PSD which has a modest efficiency of around 5×10-3 12

13. 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

14. 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. 510-18 cm2
Less efficient than C6H6 desorption due to strong hydrogen bonding!

15. 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! 15

16. 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? 16

17. 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,

18. 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

19. 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!

20. 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

21. 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,
Efficient Hydrogen Production from Electron Radiolysis of Cyclohexane as Model of a Hydrogen-rich Carbonaceous Interstellar Ice
K. A. K. Gadallah, D. Marchione, S. P. K. Koehler and M. R. S. McCoustra, Phys. Chem. Chem. Phys., Submitted

22. 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 as CO binding energy is slightly higher (8 – 12 versus kJ mol-1) and the exciton-mediated desorption channel seen in H2O 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!

23. 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 ).