1. Helping to Redefine the Onion?
Surface Science Investigations of Physics and Chemistry at Icy Interfaces
Helping to Redefine the Onion?
Martin McCoustra
Alexander Rosu-Finsen, Demian Marchione, Ali Abdulgalil, John Thrower and Mark Collings

2. The Chemically-controlled Cosmos
Surface Science can help us to understand some of the complex processes occurring on icy surfaces from dust grains to icy moons; especially relating to thermal and non-thermal mechanisms for desorption!

3. The Chemically-controlled Cosmos
H2O
Silicate or Carbon Core CO
The Dusty Onion… H2O is reactively accreted first to form a H2O-rich layer, as the dust further cools CO begins to deposit and H atom reactions and photochemistry yield CH3OH and CO2 respectively in that CO-rich layer. Energetic and thermal processing produces complex organic molecules (COMs) and a refractory organic outer layer.

4. Surface Processes on Grains
The core of this presentation aims to present a number of recent experiments that may help shed light on the structure of icy grain mantles and their compositional evolution with time
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., Submitted
Electron-promoted Desorption of H­2O from Water Ice Surfaces
A. G. G. M. Abdulgalil, A. Rosu-Finsen, D. Marchione, J. D. Thrower, M. P. Collings
and M. R. S. McCoustra, Phys. Chem. Chem. Phys., Submitted
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, 4

5. Water Diffusion on Silica
Simple Experiment
Ballistic deposition of H2O on amorphous silica at temperatures below 20 K
TPD consistent with zero order kinetics which suggests that H2O de-wets from the silica surface
When does this process begin? 5

6. Water Diffusion on Silica
A simple RAIRS experiment
Ballistic deposition of sub-monolayer quantity of H2O and anneal ice film and observe what happens
Intensity of O-H stretching band increases with annealing temperature in a regime where there is no increase in the number of H2O on the surface!
Diffusion of isolated H2O and small clusters of H2O into bulk islands on the silica! 6

7. Water Diffusion on Silica
Some undergraduate kinetics…
Fix temperature and measure RAIR spectrum as a function of (annealing) time
First order kinetics analysis straight from the undergraduate laboratory
18 K 7

8. Water Diffusion on Silica
Some undergraduate kinetics…
Fix temperature and measure RAIR spectrum as a function of (annealing) time
First order kinetics analysis straight from the undergraduate laboratory
Ea ≈ 2 kJ mol-1
Ea ≈ 0 kJ mol-1
T ≈ 25 K
Arrhenius analysis as a function of annealing temperature 8

9. Water Diffusion on Silica
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 9

10. Water Diffusion on Silica
Water 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!

11. Non-thermal Desorption of Water Ice
H2O is the dominant component of interstellar and planetary ices
H2O absorbs strongly at the red end of the optical spectrum via overtone and combination bands of the vibrational fundamentals of the H2O molecule which explains why ice appears blue
Water Absorption

12. Unsaturate Absorption
Non-thermal Desorption of Water Ice
Increasing unsaturation in organic molecules promotes strong absorption from the near-UV, violet and blue end of the optical spectrum; the more unsaturation the further into the visible the absorption occurs
Water Absorption
Unsaturate Absorption

13. Non-thermal Desorption of Water Ice
The interstellar radiation field extends across the electromagnetic spectrum but is at its strongest in the region extending from the near-UV to the mid-IR
Might therefore expect significant UV-Visible photon-induced desorption in molecularly contaminated ices in addition to the expected VUV-induced desorption
J. S. Mathis, P. G. Mezger, and N. Panagia, Astron. Astrophys., 1983, 128, 212.

14. Non-thermal Desorption of Water Ice
VUV photon-induced desorption of H2O has a modest efficiency of around 5×10-3 (M. S. Westley, R. A. Baragiola, R. E. Johnson and G. A. Baratta, Planet. Space Sci., 1995, 43, )

15. Non-thermal Desorption of Water Ice
Model comprising of a layer of amorphous solid water (ASW) with benzene (C6H6) on top
C6H6 is the prototypical aromatic hydrocarbon and such species represent the major sink for galactic carbon
Near zero order thermal desorption (TPD) of C6H6 at all sub-monolayer exposures suggests island film growth with some isolated C6H6 between the islands
J. D. Thrower, M. P. Collings, F. J. M. Rutten, and M. R. S. McCoustra, J. Chem. Phys., 2009, 131, 15

16. Non-thermal Desorption of Water Ice
In RAIRS, a small reduction in C6H6 ring breathing frequency is consistent with donation of  electron density to an electrophilic centre
C6H6 interacts with the H2O surface via weak  hydrogen bonding 16

17. Non-thermal Desorption of Water Ice
J. D. Thrower, M. P. Collings, F. J. M. Rutten & M. R. S. McCoustra, Mon. Not. Royal Astron. Soc., 2009, 394, 1510; J. D. Thrower, M. P. Collings, F. J. M. Rutten & M. R. S. McCoustra, J. Chem. Phys., 2009, 131,
Photons can induce desorption
Directly where the absorbing species itself desorbs
Indirectly in which energy transfer and relaxation processes in the absorbing molecule promote desorption of the mechanically coupled matrix via unimolecular decomposition of the “hot” surface adsorbate-substrate cluster

18. Non-thermal Desorption of Water Ice
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! 18

19. Non-thermal Desorption of Water Ice
Cosmic rays are predominantly protons
The distribution peaks at an energy of around 100 MeV
Proton scattering from the electrons in the ice is the dominant energy loss mechanism and produces a distribution of secondary electrons that peaks in the 100 to 500 eV range; each proton producing at least 100 electrons
Water ice supports a long-lived excitation at an energy of around eV; each 100 eV electron can produce excitations
C. J. Shen, J. M. Greenberg, W. A. Schutte, and E. F. van Dishoeck, Astron. Astrophys, 2004, 415, 203

20. Non-thermal Desorption of Water Ice
H2O EPD in this energy range was investigated a combination of TPD and RAIRS (looking only at total loss and not what’s lost) and found to be ca. 510-18 cm2

21. Photon Flux at ca. 250 nm ≈ 108 cm-2 s-1
A Toy Astrophysical Model
Non-thermal desorption of ices mediated by
Photon-stimulated desorption involving photons from the interstellar radiation field
J. S. Mathis, P. G. Mezger, and N. Panagia, Astron. Astrophys., 1983, 128, 212.
Photon Flux at ca. 250 nm ≈ 108 cm-2 s-1

22. Limiting cosmic ray induced UV Flux in Dense Regions ≈ 103 cm-2 s-1
A Toy Astrophysical Model
C. J. Shen, J. M. Greenberg, W. A. Schutte, and E. F. van Dishoeck, Astron. Astrophys, 2004, 415, 203
Limiting cosmic ray induced UV Flux in Dense Regions ≈ 103 cm-2 s-1
Non-thermal desorption of ices mediated by
Photon-stimulated desorption involving photons from the interstellar radiation field
Photon-stimulated desorption involving the background VUV field produced by cosmic ray ionisation

23. A Toy Astrophysical Model
For 1 MeV cosmic ray protons, the secondary electron yield is around 90 cm-2 s-1 at 100 to 300 eV
C. J. Shen, J. M. Greenberg, W. A. Schutte, and E. F. van Dishoeck, Astron. Astrophys, 2004, 415, 203
Non-thermal desorption of ices mediated by
Photon-stimulated desorption involving photons from the interstellar radiation field
Photon-stimulated desorption involving the background VUV field produced by cosmic ray ionisation
Electron-stimulated desorption associated from secondary electrons produced by cosmic ray interactions with icy grains

24. A Toy Astrophysical Model
Kinetic simulations based on the assumptions of photon and electron fluxes on the previous slides

25. A Toy Astrophysical Model
Kinetic simulations based on the assumptions of photon and electron fluxes on the previous slides

26. A Toy Astrophysical Model
We can add re-adsorption to this simulation and use our model to look at simple systems approaching equilibrium if we ignore thermal desorption
From the gas phase
From the solid state 26

27. A Toy Astrophysical Model
We can add re-adsorption to this simulation and use our model to look at simple systems approaching equilibrium if we ignore thermal desorption
From the gas phase
From the solid state
A. G. M. Abdulgalil, M. P. Collings and M. R. S. McCoustra, Mon. Not. Roy. Astron. Soc., in preparation
This allows us to investigate the dependence of the equilibrium on Av 27

28. A Toy Astrophysical Model
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?
Photodissociation is the most efficient destruction mechanism but the products (H and OH) are likely recycled into the ice phase with high efficiency
CO Overlayer capping?
Capping will suppress desorption but the efficiency, the inelastic mean free path for desorption, is not known though it might be estimated from molecular dynamics simulations and is open to experimental study 28

29. Non-thermal Desorption of Water Ice
Electron-promoted desorption is much more efficient at removing H2O from surfaces in cold dense environments than VUV photons. Both pale in comparison to UV-Visible photon-stimulated desorption at low extinction.

30. Delaying Organic Accumulation
Icy films of C6H6 on H2O ice were irradiated with electrons of energies of between 100 and 400 eV
Desorption of C6H6 mediated by the H2O ice and the formation of excitons
Desorption of C6H6 diffusing between islands has a massive cross-section of around 210-15 cm2 in this range cf. 510-18 cm2 for H2O
We see no evidence for any chemical transformations only desorption
J. D. Thrower, M. P. Collings, F. J. M. Rutten, and M. R. S. McCoustra, Chem. Phys. Lett., 2011, 505, 30

31. Delaying Organic Accumulation
Two questions...
Is the high cross-section a consequence of the presence of the hydrogen bond network in solid H2O?
How is the excitation transferred from the point of its creation to the surface-adsorbate complex?

32. Addressing the First Question…
In H2O, low energy electron scattering produces excitations originating from states rich in O character
Similar states exist in methanol (CH3OH) and dimethyl ether (CH3OCH3)
Repeating our electron-promoted desorption studies on these substrates will allow us to address the first of these questions
G. A. Kimmel, T. M. Orlando, C. Vézina, and L. Sanche, J. Chem. Phys., 11994, 101,

33. Addressing the First Question…
But first…
Does C6H6 behave on CH3OH and CH3CH2OCH2CH3 (diethyl ether as our based temperature is restricted to 110 K and dimethyl ether will not condense) as it does water?
RAIRS shows the interactions are weaker than that of C6H6 and H2O

34. Addressing the First Question…
But first…
Does C6H6 behave on CH3OH and CH3CH2OCH2CH3 (diethyl ether as our based temperature is restricted to 110 K and dimethyl ether will not condense) as it does on H2O?
RAIRS shows the interactions are weaker than that of C6H6 and H2O

35. Addressing the First Question…
Hint of a fast desorption from CH3OH (red)
Linear hydrogen bonded chains
CH3OH has no dangling OH groups at the surface so CH3OH must re-orientate on surface if C6H6 is to π hydrogen bond to the surface and the long time rise in signal is probably due to that!
No evidence for fast process on (CH3CH2)2O
No intermolecular hydrogen bonding
C6H6 interacts with the (CH3CH2)2O via van der Waals forces

36. Addressing the First Question…
Hydrogen bonding is crucial
For transporting excitation to the interface
Providing a dissociation coordinate between the bulk hydrogen bonded network and the terminal hydrogen bonded group (C6H6 in this instance)
Key question remains as to the mechanism of the excitation transport
Resonant Energy Transfer cf. Förster
Excited State Proton Transfer cf. Dexter
Looking for funding to develop an experiment!

37. A Toy Astrophysical Model
On-going kinetic simulations of CO film growth in a cooling environment based on the assumptions of VUV photon and electron fluxes on the previous slides
Remember secondary electron promoted desorption is some 60 to 70 times more efficient than VUV photon-stimulated desorption!

38. Delaying Organic Accumulation
Electron-promoted desorption is fastest from H2O surfaces for adsorbates weakly bound to the hydrogen-bonding network. CO does this and could therefore be subject to this desorption mechanism, delaying the accumulation of organics on and in H2O compared to silica or carbon surfaces!

39. Conclusions
H2O will de-wet 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 and accelerate it in warming environments
Highly efficient CO desorption induced by CR secondary electrons could potentially delay CO accretion (and hence organic formation) on H2O surfaces
CO accretion (and hence organic formation) on silica surfaces may be favoured compared to H2O surfaces as CO binding energy is slightly higher (8.2 – 12.2 versus 10 kJ mol-1) and the exciton-mediated desorption channel seen in H2O does not operate!

40. 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 ££
Framework 7
EPSRC and STFC
Leverhulme Trust
University of Nottingham
Heriot-Watt University
This research was (in part) funded by the LASSIE Initial Training Network, which is supported by the European Commission's 7th Framework Programme under Grant Agreement No