1. Cosmic Rays, Electrons and Photons… A Common Foundation
Martin McCoustra

2. The Chemically-controlled Cosmos
NGC 3603
W. Brander (JPL/IPAC), E. K. Grebel (University of Washington) and Y. -H. Chu (University of Illinois, Urbana-Champaign)
Diffuse ISM
Dense Clouds
Star and Planet Formation
(Conditions for Evolution of Life and Sustaining it)
Stellar Evolution and Death

3. The Chemically-controlled Cosmos
Icy
Mantle
Silicate or Carbonaceous Core nm CO N2
H2O
NH3
Heat
Input
Thermal
Desorption
CH3OH
CO2
CH3NH2
UV / X-ray
Input
Photodesorption
Cosmic Ray
Input
Sputtering and Exciton-promoted Desorption

4. The Chemically-controlled Cosmos
Processing of icy grains by photons (Visible, UV, VUV and higher energy) and cosmic radiation (high-energy charged particles) is a crucial process for increasing the chemical complexity of the Universe… but it has a common foundation.

5. Photons – Ionising versus Non-ionising
Interstellar radiation field is subject to environmental constraint
Short (VUV) wavelengths, peaking around Lyman  and typically from H+/H2+-electron recombination, are scattered more readily but plateau inside dense objects
Optical (UV-Visible-Near IR) wavelengths, peaking in the yellow and associated with stellar blackbody radiation, are less readily scattered
Thermal (Far-IR) wavelengths, from dust emission peaking around 100 m, are weakly scattered
J. S. Mathis, P. G. Mezger, and N. Panagia, Astron. Astrophys., 1983, 128, 212. 5

6. Photons – Ionising versus Non-ionising
Can’t rely on gas-phase spectra
Solid state effects including lowering of ionisation energies, stabilisation of dissociative repulsive states, positional order versus disorder, dipole ordering…
So we need to measure absorption cross-section (abs) versus wavelength extending into visible and near-IR
Mason, Muñoz Caro, Chen, Brown…
Have to address issue of what happens to electronic excitation
Branching ratios (quantum yields) for ionisation, luminescence, relaxation, chemistry…
Chemistry may be excited state specific
J. Lasne, Y. –J. Chen, D. Field, A. M. Cassidy, M. R. S. McCoustra,, G. M. Muñoz Caro, Nature Chemistry, in preparation 6

7. Photons – Ionising versus Non-ionising
In terms of photon-promoted desorption can typically identify three processes from existing work by the likes of McCoustra, Fillion… on insulating supporting substrates
Direct Adsorbate Desorption
Adsorbate-mediated desorption – an absorbing adsorbate promotes volatile substrate desorption by energy transfer
Substrate-mediated desorption – an absorbing substrate promotes desorption of an adsorbate by energy transfer
Conducting surfaces (e.g. graphite, graphene…) more complex
Hot electron and photoelectron chemistry as demonstrated by Chakarov…
J. D. Thrower, M. P. Collings, and M. R. S. McCoustra, D. J. Burke and W. A. Brown, A. Dawes, P. D. Holtom, P. Kendall, and N. J. Mason, F. Jamme, H. J. Fraser, I. P. Clark and A. W. Parker, J. Vac. Sci. Technol. A, 2008, 26, 919 7

8. Photons – Ionising versus Non-ionising
Rate constants for desorption and ionisation can be represented as integrals over the wavelength dependence and can be written in terms of relevant cross-sections or quantum yields (branching ratios) 8

9. Photons…
Branching ratios are a crucial missing link in the list of things we need to know in photophysics and photochemistry on and in ices.

10. Cosmic Rays…
Cosmic rays are predominantly protons with an energy distribution peaking at around 100 MeV
Heavier nuclei and high energy electrons make up the balance of the flux
Secondary electrons produced by photon interactions (i.e. electrons that are scattered and lose energy as they try to escape the solid) can have substantial energies and can be considered here
C. J. Shen, J. M. Greenberg, W. A. Schutte, and E. F. van Dishoeck, Astron. Astrophys, 2004, 415, 203 10

11. Cosmic Rays… Electrons and Excitons
Charged particles scatter in solids and lose energy
by dipole scattering (promotes electronic excitation including ionisation until particle energy is typically less than few eV when vibration excitation is preferred)
Impact Scattering
Negative Ion Resonances (electron scattering)
Charged particles including secondary electrons are eventually thermalised in the solid if there’s enough solid
Secondary excitations are like photon excitations (excitons)
One particle… lots of secondary electrons and excitations
C. J. Shen, J. M. Greenberg, W. A. Schutte, and E. F. van Dishoeck, Astron. Astrophys, 2004, 415, 203 11

12. Cosmic Rays… Electrons and Excitons
Excitons are the fundamental promoter of non-thermal physics (desorption) and chemistry (chemical transformation) in H2O-rich ices
Mediator
Photon (Non-ionisng)
Exciton (1)
Photon (Ionising)
Exciton (1 – 10s)
CR (MeV)
Exciton (100s – 1000s)
Sputtering
CR (GeV/TeV)

13. Cosmic Rays… Electrons and Excitons
The common foundations in photon and charged particle driven physics and chemistry are electronic excitation (exciton) induced physics and chemistry and thermal electron induced physics and chemistry.

14. Physics and Chemistry with Excitons
Excitons are electronic excitations
Water excitons are at energies of 8 to 14 eV, are relatively long-lived and mobile
Example system
C6H6 on compact ASW at 110 K
TPD tells us C6H6 doesn’t wet ASW so undergoes island growth
RAIRS tells us we have a weak  hydrogen bond interaction between C6H6 and ASW
Two processes observed as a function of coverage
At sub-monolayer coverage, fast desorption of isolated C6H6 from the ASW which has a massive cross-section of around 210-15 cm2
At higher coverage, slower process reflecting C6H6 desorption from solid C6H6 with a cross-section of 510-17 cm2
D. Marchione, J. D. Thrower, and M. R. S. McCoustra, Phys. Chem. Chem. Phys., 2016, 18, 4026 14

15. Blue – Amorphous Silica
Water… An Aside
If water grows by forming three dimensional islands, a key question is how mobile are water molecules at cryogenic temperatures.
A laboratory study
Sub-monolayer quantities of H2O on amorphous silica and graphite at temperatures from 18 K
RAIRS study of OH stretching band intensity with time and surface temperature
Two Arrhenius components
Below 25 K, 2 kJ mol-1 barrier for H2O diffusion over the silica surface
Above 25 K to around 110 K, barrier drops to 0 kJ mol-1 and is associated with hydrogen-bond network reconnection
Blue – Amorphous Silica
Red - Graphite
A. Rosu-Finsen, D. Marchione, T. L. Salter, J. W. Stubbing, W. A. Brown, and M. R. S. McCoustra, Phys. Chem. Chem. Phys., 2016, 18, 31930 15

16. Water… An Aside
Water diffusion into three dimensional islands is facile on laboratory timescales at temperatures as low as 18 K. In interstellar environments, water will clump together on grain surfaces leaving bare grain surface.

17. Physics and Chemistry with Excitons
What about other surfaces?
Example systems
C6H6 on CH3OH and CH3CH2OCH2CH3 at 110 K
TPD tells us C6H6 doesn’t wet CH3OH but wets CH3CH2OCH2CH3
RAIRS tells us we have a weak  hydrogen bond interaction between C6H6 and CH3OH
Different from C6H6-ASW system
Little or no water-like exciton- promoted fast C6H6 desorption in the CH3OH system, except initially and as we induced dangling OH production by molecular reorientation, only the slower process typical of desorption from a molecular layer
No water-like exciton-promoted fast C6H6 desorption in the CH3OH system at all; only desorption from a molecular layer
A – CH2CH3OCH2CH3
B – CH3OH
C – H2O
D. Marchione,and M. R. S. McCoustra, Phys. Chem. Chem. Phys., 2016, 18, 29747 17

18. Physics and Chemistry with Excitons
At low coverage (sub-monolayer), weakly hydrogen-bonded adsorbates on water ice surfaces are likely to be subject to highly efficient exciton-promoted desorption.

19. Physics and Chemistry with Excitons
What about exciton-promoted chemistry?
Let’s compare 1 MeV H+ with 250 eV electrons by looking at CH3CN
Both experiments measure total loss but…
Proton experiment sees only bulk chemistry no material loss with a cross-section of 1.5×10-15 cm2
Electron experiment sees only selvedge loss (as what we don’t know) with a cross-section of 4×10-18 cm2
These are consistent if we assume a selvedge plus bulk model
A. G. M. Abdulgalil, D. Marchione, J. D. Thrower, M. P. Collings, M. R. S. McCoustra, F. Islam, M. E. Palumbo, E. Congiu and F. Dulieu, Phil. Trans. Roy. Soc. A, 2013, 371, 19

20. Physics and Chemistry with Excitons
Another example of exciton-promoted chemistry?
Let’s look at water
Simplest chemistry is H atom formation and hence H2 formation
Use H2O (0 – 15 nm)/D2O (0.2 nm)/ H2O (40 nm) layered ASW at 110 K for incident electrons at 450 eV
Detect significant H2/HD/D2 production
HD/D2 production delayed as D2O layer goes deeper into the film
A – H3
B – HD
C – D2
K. A. K. Gadallah, D. Marchione, S, P. K. Koehler and M. R. S. McCoustra, Phys. Chem. Chem. Phys., 2017, 19, in print 20

21. Physics and Chemistry with Excitons
Another example of exciton-promoted chemistry?
Let’s look at water
Simplest chemistry is H atom formation and hence H2 formation
Use H2O (0 – 15 nm)/D2O (0.2 nm)/ H2O (40 nm) layered ASW at 110 K for incident electrons at 450 eV
Detect significant H2/HD/D2 production
HD/D2 production delayed as D2O layer goes deeper into the film
HD/D2 production also electron energy dependent for fixed D2O layer depth peak production occurring at shorter times with increasing electron energies (black 400 eV, red 450 eV and blue 500 eV)
A – H3
B – HD
C – D2
K. A. K. Gadallah, D. Marchione, S, P. K. Koehler and M. R. S. McCoustra, Phys. Chem. Chem. Phys., 2017, 19, in print 21

22. Physics and Chemistry with Excitons
Contrast between 1 MeV H+ and 250 eV electron irradiation of CH3CN and observation of H2 formation at depth in a layered and labelled H2O films which suggests, at the very least, we need to consider a selvedge plus bulk model for physics and chemistry in thin solid films.

23. Physics and Chemistry with Excitons
What about other systems?
Back to CH3OH and CH3CH2OCH2CH3
Simplest chemistry is H atom formation and hence H2 formation
H2O / CH3OH / CH3CH2OCH2CH3 layers with a sub-monolayer amount of C6H6 at 110 K with 250 eV electrons
Detect H2 production by QMS
Negligible H2 from C6H6
Significantly more H2 production from the organic films
In addition to O excitation (which is linked into the H2O hydrogen bond network and promotes desorption), organics open C excitation pathways (which promotes dissociation)
A , A’– H2O
B, B’ – CH3OH
C, C’ – CH3CH2OCH2CH3
D. Marchione and M. R. S. McCoustra, ACS Planet. Space Chem., 2017, submitted 23

24. Physics and Chemistry with Excitons
In modelling desorption and other processes involving cosmic rays, rate constants for desorption and other processes can be represented as integrals over the cosmic ray distribution and secondary electron distribution and can be written in terms of relevant cross-sections or quantum yields (branching ratios) 24

25. Physics and Chemistry with Excitons
Branching ratios are a crucial missing link in the list of things we need to know in exciton-promoted physics and chemistry on and in ices.
Electron scattering cross-sections from surfaces are also a necessary input to the list of things we need to know in exciton-promoted physics and chemistry on and in ices.

26. Thermalised Electrons…
The question of what happens with thermalised (meV) electrons remains open in terms of physics and chemistry.

27. Acknowledgements Dr.’s Mark Collings and Jerome Lasne
Vicky Frankland, Rui Chen, John Dever, Simon Green, John Thrower, Ali Abdulgalil, Demian Marchione and Alex Rosu-Finsen, Skandar Taj
Various Colleagues across the Globe! ££
Framework 7 and H2020
EPSRC and STFC
Leverhulme Trust
University of Nottingham
Heriot-Watt University
This research was (in part) funded by the LASSIE Initial Training Network, which was supported by the European Commission's 7th Framework Programme under Grant Agreement No