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
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
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!
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
The Chemically-controlled Cosmos
Silicate or Carbon Core
H2O
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.
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
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
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
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?
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Water Diffusion on Silica
 A simple RAIRS experiment
 Ballistic
deposition
of
submonolayer 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!
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Water Diffusion on Silica
undergraduate
 Fix temperature and measure RAIR
spectrum as a function of
(annealing) time
 First order kinetics analysis straight
from the undergraduate laboratory
0.007
0.006
0.005
log(R0/R)
 Some
kinetics…
0.004
0.003
0 ML H2O
18 K
0.5 ML H2O
1 hour
2 hours
3 hours
4 hours
5 hours
A infinity
0.002
0.001
0.000
3600
3500
3400
3300
3200
-1
Wavenumber (cm )
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
3100
Water Diffusion on Silica
undergraduate
 Fix temperature and measure RAIR
spectrum as a function of
(annealing) time
 First order kinetics analysis straight
from the undergraduate laboratory
 Arrhenius analysis as a
function
of
annealing
temperature
T ≈ 25 K
Ea ≈ 0 kJ mol-1
-6
-7
Avg of ln(k)
 Some
kinetics…
Ea ≈ 2 kJ mol-1
-8
-9
-10
-11
0.01
0.02
0.03
0.04
-1
1/Temp (K )
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
0.05
0.06
Water Diffusion on Silica
analysis
 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)
T ≈ 25 K
Ea ≈ 0 kJ mol-1
-6
-7
Avg of ln(k)
 Arrhenius
suggests…
Ea ≈ 2 kJ mol-1
-8
-9
-10
-11
0.01
0.02
0.03
0.04
-1
1/Temp (K )
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
0.05
0.06
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!
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
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
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
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
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
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 photoninduced
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.
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
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)
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
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
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
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
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Non-thermal Desorption of Water Ice
 Photons
desorption
can
induce
 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
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.
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
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!
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
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 longlived 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
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
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
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
A Toy Astrophysical Model
 Non-thermal desorption
ices mediated by
of
 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
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
A Toy Astrophysical Model
 Non-thermal desorption
ices mediated by
of
 Photon-stimulated
desorption
involving
photons
from
the
interstellar radiation field
 Photon-stimulated
desorption
involving the background VUV field
produced by cosmic ray ionisation
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
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
A Toy Astrophysical Model
 Non-thermal desorption
ices mediated by
of
 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
C. J. Shen, J. M. Greenberg, W. A. Schutte,
and E. F. van Dishoeck, Astron. Astrophys,
2004, 415, 203
For 1 MeV cosmic ray protons, the
secondary electron yield is around 90
cm-2 s-1 at 100 to 300 eV
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
A Toy Astrophysical Model
ΔT
H2O(s) 
 H2O(g)
-
dnH2O(s)
dt
 ν des e Edes /RT
h ISRF
H2O(s) 
 H2O(g)
 Kinetic simulations based on
the assumptions of photon
and electron fluxes on the
previous slides
-
dnH2O(s)
dt


  fISRF ( ) ISRF ( ) nH2O(s)


h CRI
H2O(s) 
 H2O(g)
-
dnH2O(s)
dt


  fCRRF( ) CRRF( ) nH2O(s)


eCRI
H2O(s) 
H2O(g)
-
dnH2O(s)
dt


  fCRIE(E ) des,CRIE(E ) nH2O(s)
E

Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
A Toy Astrophysical Model
 Kinetic simulations based on
the assumptions of photon
and electron fluxes on the
previous slides
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
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
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
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
 This allows us to investigate
the dependence of the
equilibrium on Av
A. G. M. Abdulgalil, M. P. Collings and
M. R. S. McCoustra, Mon. Not. Roy.
Astron. Soc., in preparation
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
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
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
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 photonstimulated desorption at low extinction.
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
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
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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 surfaceadsorbate complex?
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
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
electronpromoted 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
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
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
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
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
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
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 reorientate 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
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
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!
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
A Toy Astrophysical Model
CO(g)  CO(ads)
dnCO(ads)
 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 photonstimulated desorption!
dt
kBT
nCO ( g )
2m
S
T
CO(s) 
H2O(g)
-
dnCO(ads)
dt
  CO(ads)e
E des,CO / RT
nCO(ads)
h CRI
CO(ads) 
 CO(g)
-
dnCO(ads)
dt


  fCRRF( ) CRRF( ) nCO(ads)


-
eCRI
CO(ads) 
CO(g)
-
dnCO(ads)
dt


  fCRIE(E ) des,CRIE(E ) nCO(ads)
E

Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
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!
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
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!
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
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.
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University