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

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!

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.

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, 4026-4034 4

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

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

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

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

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

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!

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

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

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.

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, 1311-1315)

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, 244711 15

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

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, 244711. 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

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

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 10 - 12 eV; each 100 eV electron can produce 8 - 10 excitations C. J. Shen, J. M. Greenberg, W. A. Schutte, and E. F. van Dishoeck, Astron. Astrophys, 2004, 415, 203

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

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

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

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

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

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

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

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

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

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.

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, 106-111 30

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?

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, 3282-3286

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

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

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

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!

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!

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!

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!

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. 238258.