Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University Photons, Electrons and Desorption An Application of Laboratory

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


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


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



CH4

IcyMantle


H

H2

H

O

H2O

H

N

H3N

Silicate or Carbonaceous Core

1 - 1000 nm

CO, N2

CO, N2



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


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 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


The Experimental Arrangement


The Experimental Arrangement


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



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


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



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


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


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.





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


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

For 1MeV cosmic ray protons, the secondary electron yield is around 90

cm-2 s-1 at 100 to 300 eV



Photon-stimulated desorption involving the background VUV field produced by cosmic ray ionisation


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


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


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


Steady-state


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


Thermal desorption


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


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

Documents

Department of Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University Photons, Electrons and Desorption An Application of Laboratory