Electronic Excitation Transport
in Ices: A Key Role for
Hydrogen Bonding
Martin McCoustra
John Thrower and Demian Marchione
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
The Chemically-controlled Cosmos
Diffuse ISM
NGC 3603
W. Brander (JPL/IPAC), E. K. Grebel (University of
Washington) and Y. -H. Chu (University of Illinois, UrbanaChampaign)
Dense Clouds
Star and Planet Formation
(Conditions for Evolution of Life
and Sustaining it)
Stellar Evolution and Death
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
The Chemically-controlled Cosmos
1 - 1000 nm
Heat
Input
CH3NH2 CH OH
3
NH3
Silicate or
Carbonaceous Core
H2O
Thermal
Desorption
CH4 CO
2
Cosmic Ray
Input
Icy
Mantle
N2
CO
Photodesorption
Sputtering and Electronstimulated Desorption
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
UV Light
Input
The Chemically-controlled Cosmos
Processing of icy grains by cosmic radiation (highenergy charged particles) is a crucial process for
increasing the chemical complexity of the Universe…
but the surface and solid state physics and chemistry
of these ices are poorly understood. This especially
true of the competition between ice desorption and
chemical transformation.
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
The Chemically-controlled Cosmos
 Cosmic
rays
are
predominantly protons
 The distribution peaks at an
energy of around 100 MeV
 Proton interaction with the
interstellar gas produces
Lyman α radiation; each
proton
producing
many
photons
 Proton interactions with ice
produce a distribution of
secondary electrons in ice
that peaks in the 100 to 500
eV range; each proton
producing many electrons
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
The Chemically-controlled Cosmos
Evidence for non-thermal processes in the cold,
dense interstellar medium is found in observations of
such environments and can be driven by cosmic ray
generated VUV photons and secondary electrons.
The key question is which is more important!
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Physics versus Chemistry on Ices
 We have previously reported
efficient
electron-promoted
desorption of benzene (C6H6)
from a layer of amorphous
solid water (ASW)
 Water is the predominant
component of interstellar ice
 C6H6 is the prototypical
aromatic hydrocarbon and
such species represent the
major sink for galactic carbon
 Zero order TPD of C6H6 at all
sub-monolayer
exposures
suggests island film growth
with some isolated C6H6
between the islands
 Efficient
associated
C6H6!
process
is
with
isolated
J. D. Thrower, M. P. Collings, F. J. M. Rutten,
and M. R. S. McCoustra, Chem. Phys. Lett.,
2011, 505, 106-111
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Physics versus Chemistry on Ices
 Small reduction in C6H6 ring
breathing
frequency
is
consistent with donation of 
electron
density
to
a
electrophilic centre
 C6H6 interacts with the water
surface via a weak 
hydrogen bond
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Physics versus Chemistry on Ices
 Desorption of isolated C6H6
has a cross-section of ca.
210-15 cm2 in this range cf.
510-18 cm2 for H2O
 Desorption from the C6H6
islands and bulk C6H6 has a
cross-section of 510-17 cm2
 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
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Exciton Transport via Hydrogen Bonds…
 Water ice supports long-lived
excitations at an energy of
around 8 - 14 eV; each 100
eV electron can produce 8 –
10 excitations
 Excitations originate 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 tell
us if hydrogen bonding is
important
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
Exciton Transport via Hydrogen Bonds…
 But first…
 Does C6H6 behave on
CH3OH and (CH3CH2)2O
(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
 TPD suggests C6H6/ behaves
on CH3OH as it does on H2O
but (CH3CH2)2O wets C6H6
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Exciton Transport via Hydrogen Bonds…
 But first…
 Does C6H6 behave on
CH3OH and (CH3CH2)2O
(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
 TPD suggests C6H6/ behaves
on CH3OH as it does on H2O
but (CH3CH2)2O wets C6H6
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Exciton Transport via Hydrogen Bonds…
 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
 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
Exciton Transport via Hydrogen Bonds…
 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
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
A Hint of Interesting Things to Come…
 Looking at H2 evolution in
each system under electronirradiation
 Predominantly see H2 from the
substrate and only a hint in the
H2O system of H2 from fast
desorbing C6H6
 More H2 released from the Ccentred species
 Is this hinting at different
behaviours for O and C
centres,
especially
in
hydrogen bonding networks?
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Astronomical Impact…
 Electron-promoted desorption (EPD; σ typically 10-18 – 10-17
cm2) is more efficient than VUV photon-stimulated desorption
(PSD; σ typically 10-22 – 10-21 cm2) in cold, dense environments
and could account for observations of molecules in such
regions
 Fast EPD may slow accumulation of species hydrogen bonding
to H2O surfaces especially CO in turn causing segregation of
CO on to the grain surface and delaying formation of complex
organics on the H2O surface
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Acknowledgements
Dr. Mark Collings
Dr. Jerome Lasne
Vicky Frankland, Rui Chen, John Dever, Simon Green,
John Thrower, 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