Download Photons, Electrons and Desorption - Heriot

Photons, Electrons and Desorption
An Application of Laboratory Surface
Science in Astrophysics
Martin McCoustra
Department of Chemistry, 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
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
The Chemically-controlled Cosmos
 At the most important part of the matter cycle in the Universe
today, chemistry exerts a controlling influence since molecules
 Maintain the current rate of star formation
 Ensure the formation of small, long-lived stars such as our own Sun
 Seed the Universe with the chemical potential for life
 But ...
 There have been problems in comparing the results of chemical
network simulations of the evolution of dense gas clouds with
observed column densities for even relatively simple species
like H2
 Chemical reactions occurring on dust grains are used to
account for the discrepancy between observations and gasphase only models of the chemical evolution of dense clouds
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
The Chemically-controlled Cosmos
H2
1 - 1000 nm
H
Icy
Mantle
Silicate or
Carbonaceous Core
H2O
H3N
H
H
CH4
CO, N2
O
N
CO, N2
Department of Chemistry, 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
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
UV Light
Input
The Chemically-controlled Cosmos
Returning molecules to the gas phase from the icy
grain mantles is an important step in the surface
physics and chemistry of grain – thermal and nonthermal mechanisms can contribute to this process.
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
A Model System
 The model system we have chosen to study is the benzenewater ice system
 C6H6 may be thought of as a prototypical PAH compound and is
amongst the list of known interstellar molecules
 Water ice is a good representation of icy mantles on grains
 C6H6 does not wet the H2O ice and forms an islanded layer; isolated
C6H6 molecules can diffuse between the islands (Ostwald ripening) at
temperatures around and above 120 K
 Amorphous silica or sapphire substrate moves us away from metal
surfaces where UV irradiation can produce lots of hot electrons that
will induce chemistry
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
The Experimental Arrangement
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
The Experimental Arrangement
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
Shining a Little Light on Icy Surfaces
 Both C6H6 and H2O are
observed
to
desorb
translationally hot (in excess
of 1000 K) in resonance with
the C6H6 absorption spectrum
around 250 nm
 Energy release can be
explained with a simple
model
of
unimolecular
decomposition
of
a
C6H6...(H2O)x surface cluster
in which C6H6 is  facially
hydrogen bonded to the
water cluster via a single H2O
molecule
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
Shining a Little Light on Icy Surfaces
 Cross-sections for C6H6 and
H2O desorption can be
estimated from PSD curves
to be 410-19 cm2 and 110-19
cm2 respectively at 250 nm
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
Shining a Little Light on Icy Surfaces
 Cross-sections for C6H6 and
H2O desorption can be
estimated from PSD curves
to be 410-19 cm2 and 110-19
cm2 respectively at 250 nm
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
Firing a Few Electrons at Surfaces
 Icy films of C6H6 and H2O ice
were irradiated with electrons
of energies of around 100 to
300 eV
 Desorption of C6H6 mediated
by the H2O ice and the
formation
of
solvated
electrons
 Desorption of C6H6 diffusing
between islands has a
massive cross-section of
around 210-15 cm2 in this
range
 Build-up and long time decay
process
associated
with
diffusion of C6H6 from islands
followed by ESD has a crosssection of 510-17 cm2
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
Firing a Few Electrons at Surfaces
 H2O ESD in this energy
range was measured by a
combination of TPD and
RAIRS to be ca. 510-17 cm2
and independent of the C6H6
coverage at exposures where
C6H6 forms islands
 Supports the idea that
electron
cooling
and
attachment to water is
important
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
Astrophysical Impact
 Non-thermal desorption
ices mediated by
of
 Photon-stimulated desorption
involving photons from the
interstellar radiation field
Photon Flux at ca. 250 nm ≈ 108 cm-2 s-1
J. S. Mathis, P. G. Mezger, and N. Panagia, Astron. Astrophys., 1983, 128, 212.
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
Astrophysical Impact
 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
Limiting cosmic ray induced UV Flux in
Dense Regions ≈ 103 cm-2 s-1
C. J. Shen, J. M. Greenberg, W. A. Schutte, and E. F. van Dishoeck, Astron. Astrophys, 2004, 415, 203
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
Astrophysical Impact
 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
For 1MeV 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
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
Astrophysical Impact
ΔT
H2O(s)  H2O(g)
dnH2O(s)
 νdes e  Edes /RT
dt
 Kinetic simulations based on
the assumptions of photon
and electron fluxes on the
previous slides
h
H2O(s) ISRF
 H2O(g)
-
dnH 2O(s)
dt


  f ISRF ( ) ISRF ( ) nH 2O(s)
 

h
H2O(s) CRI
 H2O(g)
-
dnH 2O(s)
dt


  f CRRF ( ) CRRF ( ) nH 2O(s)
 

e-CRI
H 2O(s) 
 H 2O(g)
-
dnH 2O(s)
dt


  f CRIE ( E ) des,CRIE ( E ) nH 2O(s)
E

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
Astrophysical Impact
 Kinetic simulations based on
the assumptions of photon
and electron fluxes on the
previous slides
 Steady-state
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
Astrophysical Impact
 Kinetic simulations based on
the assumptions of photon
and electron fluxes on the
previous slides
 Steady-state
 Thermal desorption
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
Conclusions
 Long wavelength ISRF-driven PSD will be important in regions
where this radiation penetrates dense molecular clouds
 ESD is as important, if not more important, than CRRF-driven
PSD in dense molecular clouds
 Surface Science techniques (both experimental and theoretical)
can help us understand heterogeneous chemistry in the
astrophysical environment
 Much more work is needed and it requires a close collaboration
between laboratory surface scientists (both experimental and
computational), chemical modellers and observers
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
Acknowledgements
John Thrower, Ali Abdulgalil and Dr. Mark Collings (Heriot-Watt)
Farah Islam and Dr. Daren Burke (UCL)
Jenny Noble and Sharon Baillie (Strathclyde)
Dr. Anita Dawes, Dr. Paul Kendall and Dr. Phil Holtom (OU)
Dr. Wendy Brown (UCL)
Dr. Helen Fraser (Strathclyde University)
Professor Nigel Mason (OU)
Professor Tony Parker and Dr. Ian Clark (CLF LSF)
££
EPSRC and STFC
University of Nottingham
££
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University