Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
Photons, Electrons and DesorptionPhotons, 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
NGC 3603W. 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
The Chemically-controlled Cosmos
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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 gas-phase only models of the chemical evolution of dense clouds
The Chemically-controlled Cosmos
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
CH4
IcyMantle
The Chemically-controlled Cosmos
H
H2
H
O
H2O
H
N
H3N
Silicate or Carbonaceous Core
1 - 1000 nm
CO, N2
CO, N2
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
The Chemically-controlled Cosmos
CH4
IcyMantle
Silicate or Carbonaceous Core
1 - 1000 nm
CO
N2
H2O
NH3
HeatInput
ThermalDesorption
UV LightInput
PhotodesorptionCosmic RayInput Sputtering and Electron-
stimulated Desorption
CH3OH
CO2
CH3NH2
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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 non-thermal mechanisms can contribute to this process.
The Chemically-controlled Cosmos
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
The model system we have chosen to study is the benzene-water 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
A Model System
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
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
Shining a Little Light on Icy Surfaces
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 410-19 cm2 and 110-19 cm2 respectively at 250 nm
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
Cross-sections for C6H6 and H2O desorption can be estimated from PSD curves to be 410-19 cm2 and 110-19 cm2 respectively at 250 nm
Shining a Little Light on Icy Surfaces
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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 210-15 cm2 in this range
Build-up and long time decay process associated with diffusion of C6H6 from islands followed by ESD has a cross-section of 510-17 cm2
Firing a Few Electrons at Surfaces
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
H2O ESD in this energy range was measured by a combination of TPD and RAIRS to be ca. 510-17 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
Firing a Few Electrons at Surfaces
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
Non-thermal desorption of ices mediated by Photon-stimulated desorption
involving photons from the interstellar radiation field
Astrophysical Impact
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
Non-thermal desorption of ices mediated by Photon-stimulated desorption
involving photons from the interstellar radiation field
Astrophysical Impact
C. J. Shen, J. M. Greenberg, W. A. Schutte, and E. F. van Dishoeck, Astron. Astrophys, 2004, 415, 203
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
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
Electron-stimulated desorption associated from secondary electrons produced by cosmic ray interactions with icy grains
Astrophysical Impact
C. J. Shen, J. M. Greenberg, W. A. Schutte, and E. F. van Dishoeck, Astron. Astrophys, 2004, 415, 203
For 1MeV cosmic ray protons, the secondary electron yield is around 90
cm-2 s-1 at 100 to 300 eV
Non-thermal desorption of ices mediated by Photon-stimulated desorption
involving photons from the interstellar radiation field
Photon-stimulated desorption involving the background VUV field produced by cosmic ray ionisation
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
Kinetic simulations based on the assumptions of photon and electron fluxes on the previous slides
Astrophysical Impact
O(g)HO(s)H 2ΔT
2
/RTEdes
deseνdt
dn- O(s)H2
O(g)HO(s)H 2h
2ISRF
O(s)HISRFISRFO(s)H
2
2 )()( nfdt
dn-
O(g)HO(s)H 2h
2CRI
O(s)HCRRFCRRFO(s)H
2
2 )()( nfdt
dn-
O(g)HO(s)H 2e
2
-CRI
O(s)HCRIEdes,CRIEO(s)H
2
2 )()( nEEfdt
dn-
E
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
Kinetic simulations based on the assumptions of photon and electron fluxes on the previous slides
Astrophysical Impact
Steady-state
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
Kinetic simulations based on the assumptions of photon and electron fluxes on the previous slides Steady-state
Astrophysical Impact
Thermal desorption
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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
Conclusions
Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University
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££
Acknowledgements
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