Characterisation of Silica and
Silicate Nano-particulate Films using
Surface Science Methodologies
Martin McCoustra
Skandar Taj, Diane Baird, Alexander Rosu-Finsen,
Victoria Frankland and Mark Collings
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
Dust Formation…
Dust is largely formed in old, reddened
AGB stars… either as soot or stoor…
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Dust Formation…
Bulk Glass or Crystal
C. R. A. Catlow, S. T. Bromley, S. Hamad, M. Mora-Fonz, A. A. Sokol and S. M. Woodley,
Phys. Chem. Chem. Phys., 2010, 12, 786.
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Dust Formation…
J. C. Weingartner and B. T. Draine, Astrophys. J., 2001, 548, 296.
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Dust Formation…
 Compositions typical of the
olivine (MgxFe2-xSiO4) and
pyroxene
(MgxFe2-xSi2O6)
families
 Depletion
 IR observations
 Rings, chains and shells to
bulk-like
materials
but
growing
in
H-rich
environment so likely surface
hydroxylated
 Space Weathered
 Competition
amorphisation
crystallisation
 Segregation
nanoparticles
 Potentially
an
catalyst
between
and
of
Fe
interesting
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Dust and Cosmic Chemistry…
H2
1 - 1000 nm
H
Icy
Mantle
Silicate or
Carbonaceous Core
H2O
H3N
H
H
CH4
CO, N2
O
N
CO, N2
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Dust and Cosmic Chemistry…
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
Dust and Cosmic Chemistry…
Surface physics and chemistry play a key role in
these processes, but the surface physics and
chemistry of grains was poorly understood. We need
to make model grains and probe their surfaces. Then
check those laboratory results against observations.
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Infrared Spectroscopy as a Probe…
Infrared Spectroscopy is a Sensitive and Versatile Probe
Line Positions
 Hooke’s Law
 k sensitive to local electronic
environment
(intermolecular
forces impact on electron
density distribution)
  sensitive to mass distribution
Line Profiles
 Fermi’s Golden Rule
  sensitive to vibrational
relaxation mechanisms when
non-radiative relaxation is
important
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Infrared Spectroscopy as a Probe…
Carbon Monoxide is a Simple and Versatile Probe Molecule
 Physisorbed CO as a probe
 Limited number of interaction geometries
 Complexes
T Complexes
B---A
C--O
O--C
A---B
O--C
C--O
A---B
 Complexes
A---B
A---B
C--O
C--O
A---B
c Complexes are the Most Common
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Infrared Spectroscopy as a Probe…
Carbon Monoxide is a Simple and Versatile Probe Molecule
 Physisorbed CO as a probe
 Limited number of interaction geometries
 CO vibrational frequency and bond length very sensitive to strength
of intermolecular forces
C Complexes
O Complexes
 Complexes
M. P. Collings, J. W. Dever and M. R. S. McCoustra, Phys. Chem. Chem. Phys., 2014, 16, 3479
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Infrared Spectroscopy as a Probe…
Carbon Monoxide is a Simple and Versatile Probe Molecule
 Physisorbed CO as a probe
 Limited number of interaction geometries
 CO vibrational frequency and bond length very sensitive to strength
of intermolecular forces
 Spectroscopically convenient and highly sensitive in buried-interface
reflection-absorption IR spectroscopy
Infrared
for RAIRS
200 – 300 nm Nanoparticle Film
Copper Substrate
H. J. Fraser, M. P. Collings and M. R. S. McCoustra, Rev. Sci.
Instrum., 2002, 73, 2161
Cool to Below 10 K
V. L. Frankland, A. Rosu-Finsen, J. Lasne, M. P. Collings, and
M. R. S. McCoustra, Rev. Sci. Inst., 2015, 86, 055103
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Infrared Spectroscopy as a Probe…
Carbon Monoxide is a Simple and Versatile Probe Molecule
 CO on silica nanoparticle film
 1 L is 10-6 mbar s exposure
and in this case around 20 L is
equivalent to a monolayer
 Recorded at instrument-limited
resolution of 0.1 cm-1 at 18 K
 CO
vibrational
frequency
consistent with observations
from Pontoppidan et al.
(Astron. Astrophys., 2003,
449,
1826)
that
are
inconsistent with CO in or on
H2O ice
 CO on grain surface in
astrophysical environments?
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Infrared Spectroscopy as a Probe…
Carbon Monoxide is a Simple and Versatile Probe Molecule
 CO on silica nanoparticle film
 CO line profile is broad and
asymmetric
 Consistent with CO in a range
of environments where the
strength of interaction of the
CO with the nanoparticle film
varies
 Can we get another handle on
that
strength
and
then
simulate the line profile?
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Understanding Interactions…
Carbon Monoxide is a Simple and Versatile Probe Molecule
 CO on silica nanoparticle film
 CO line profile is broad and
asymmetric
 Consistent with CO in a range
of environments where the
strength of interaction of the
CO with the nanoparticle film
varies
 Can we get another handle on
that
strength
and
then
simulate the line profile?
 CO temperature programmed
desorption (TPD)
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Understanding Interactions…
Carbon Monoxide is a Simple and Versatile Probe Molecule
 CO on silica nanoparticle film
 TPD data can be “inverted” to
give the distributions of CO
binding energies on the
nanoparticle film surface
 But have to assume first order
kinetics and a pre-exponential
factor, ν, consistent with
physisorption (1012 s-1)
 Gives us Edes versus Nads and
hence P(Edes) versus Edes
S. L. Tait, Z. Dohnalek, C. T. Campbell and B. D. Kay,
J. Chem. Phys., 2005, 122, 164707
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Understanding Interactions…
Carbon Monoxide is a Simple and Versatile Probe Molecule
 CO on silica nanoparticle film
 TPD data can be “inverted” to
give the distributions of CO
binding energies on the
nanoparticle film surface
 But have to assume first order
kinetics and a pre-exponential
factor, ν, consistent with
physisorption (1012 s-1)
 Gives us Edes versus Nads and
hence P(Edes) versus Edes
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Understanding Interactions…
Carbon Monoxide is a Simple and Versatile Probe Molecule
 CO on silica nanoparticle film
 TPD data can be “inverted” to
give the distributions of CO
binding energies on the
nanoparticle film surface
 But have to assume first order
kinetics and a pre-exponential
factor, ν, consistent with
physisorption (1012 s-1)
 Gives us Edes versus Nads and
hence P(Edes) versus Edes
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Understanding Interactions…
Carbon Monoxide is a Simple and Versatile Probe Molecule
 CO on silica nanoparticle film
 Assume C type interaction at
the silica
 Range of interaction energies
consistent with correlation
introduced earlier allowing
estimation of CO line position
 Two
limiting
deposition
behaviours
Δ𝜐
cm−1 = 3.330 𝐸𝑏 / kJ mol−1 + 2.308
𝜐(𝐸𝑑𝑒𝑠 = 𝜐0 + Δ𝜐(𝐸𝑑𝑒𝑠
 Ballistic
 Adsorb and Diffuse
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Simulating IR Profiles…
Carbon Monoxide is a Simple and Versatile Probe Molecule
Ballistic Deposition
No preference for any adsorption site, so
sites sampled according to P(Edes)
distribution
𝑃 𝐸𝑑𝑒𝑠 𝑒 − 𝜈−𝜐(𝐸𝑑𝑒𝑠
𝐼 𝜐 = 𝐼0
𝐸𝑑𝑒𝑠
2
2𝛿 2
Adsorption and Diffusion
The sites are sampled in energetic
preference order with the most strong
bound filling first, i.e. sites sampled
according to inverse Boltzmann weighted
P(Edes) distribution
𝑃 𝐸𝑑𝑒𝑠 𝑒 −𝐸𝑑𝑒𝑠
𝐼 𝜐 = 𝐼0
𝐸𝑑𝑒𝑠
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
𝑅𝑇 − 𝜈−𝜐(𝐸𝑑𝑒𝑠 2 2𝛿 2
𝑒
Understanding Interactions…
Carbon Monoxide is a Simple and Versatile Probe Molecule
 CO on silica nanoparticle film
 Three parameters need optimisation if the synthesised line profile is
to be compared with the experimental
 𝜐0 - CO on silica line position in the absence of any interaction, i.e.
accounting for the effect of the surface mass alone on the vibration
  - CO line width which reflects the dynamics of vibrational relaxation
 𝐼0 - Intensity scaling factor
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Simulating IR Profiles…
Carbon Monoxide is a Simple and Versatile Probe Molecule
Ballistic Deposition
Instrument-limited Linewidth
Line profile is too broad
and steeply sided even at
instrument limited
resolution to reproduce the
line profile!
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Simulating IR Profiles…
Carbon Monoxide is a Simple and Versatile Probe Molecule
Adsorption and Diffusion
Instrument-limited Linewidth
Optimised Linewidth
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Simulating IR Profiles…
Carbon Monoxide is a Simple and Versatile Probe Molecule
Ballistic Deposition on Porous Amorphous Solid Water
Optimised Linewidth
R. S. Smith, R. A. May and B. D. Kay, J. Phys. Chem. B, 2016, 120, 1979.
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Simulating IR Profiles…
Carbon Monoxide is a Simple and Versatile Probe Molecule
𝜐0 / cm-1
 / cm-1
CO on Silica NP
2076  0.5
4.00  0.25
CO on p-ASW
2108  0.5
3.00  0.25
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University
Conclusions
 IR spectroscopy of physisorbed CO provides a sensitive probe
of heterogeneous nanoparticle surface environments
 CO line profiles can be simulated from a knowledge of the
strength of the physisorption interaction as a function of the
surface density of the CO
 Opens a window onto the dynamics of the vibrational relaxation
of CO on environmentally heterogeneous insulator surfaces
 Potentially extendable to other systems that exhibit
physisorption interactions with surfaces and just might be
“invertable” if the relevant spectroscopic correlation (i.e.
interaction energy versus vibrational frequency) is known
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