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Photons, Electrons and Desorption. An Application of Laboratory Surface Science in Astrophysics. Martin McCoustra. NGC 3603 W. Brander (JPL/IPAC), E. K. Grebel (University of Washington) and Y. -H. Chu (University of Illinois, Urbana-Champaign). The Chemically-controlled Cosmos. Diffuse ISM.

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photons electrons and desorption

Photons, Electrons and Desorption

An Application of Laboratory Surface Science in Astrophysics

Martin McCoustra

slide2

NGC 3603

W. Brander (JPL/IPAC), E. K. Grebel (University of Washington) and Y. -H. Chu (University of Illinois, Urbana-Champaign)

The Chemically-controlled Cosmos

Diffuse ISM

Dense Clouds

Star and Planet Formation

(Conditions for Evolution of Life and Sustaining it)

Stellar Evolution and Death

the chemically controlled cosmos
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 gas-phase only models of the chemical evolution of dense clouds
the chemically controlled cosmos1

1 - 1000 nm

H2

H

Icy

Mantle

Silicate or Carbonaceous Core

H3N

H

H2O

H

CH4

CO, N2

N

CO, N2

O

The Chemically-controlled Cosmos
the chemically controlled cosmos2

1 - 1000 nm

Heat

Input

CH3NH2

CH3OH

Icy

Mantle

Silicate or Carbonaceous Core

NH3

H2O

Thermal

Desorption

N2

CH4

CO2

CO

Cosmic Ray

Input

Photodesorption

Sputtering and Electron-stimulated Desorption

UV Light

Input

The Chemically-controlled Cosmos
the chemically controlled cosmos3
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 non-thermal mechanisms can contribute to this process.

a model system
A Model System
  • 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
slide10

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
slide11

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
slide12

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
slide13

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 cross-section of 510-17 cm2
slide14

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
slide15

Photon Flux at ca. 250 nm ≈ 108 cm-2 s-1

Astrophysical Impact

  • Non-thermal desorption of ices mediated by
    • Photon-stimulated desorption involving photons from the interstellar radiation field

J. S. Mathis, P. G. Mezger, and N. Panagia, Astron. Astrophys., 1983, 128, 212.

slide16

Limiting cosmic ray induced UV Flux in Dense Regions ≈ 103 cm-2 s-1

Astrophysical Impact

  • 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

C. J. Shen, J. M. Greenberg, W. A. Schutte, and E. F. van Dishoeck, Astron. Astrophys, 2004, 415, 203

slide17

For 1MeV cosmic ray protons, the secondary electron yield is around 90 cm-2 s-1 at 100 to 300 eV

Astrophysical Impact

  • 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
  • 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

slide18

Astrophysical Impact

  • Kinetic simulations based on the assumptions of photon and electron fluxes on the previous slides
slide19

Astrophysical Impact

  • Kinetic simulations based on the assumptions of photon and electron fluxes on the previous slides
  • Steady-state
slide20

Astrophysical Impact

  • Kinetic simulations based on the assumptions of photon and electron fluxes on the previous slides
    • Steady-state
  • Thermal desorption
slide21

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
slide22

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

££