Presentation on theme: "Preliminary Laboratory Studies of the Photoprocessing of PAH / H 2 O Mixtures in the Interstellar Medium John Thrower Department of Chemistry, School of."— Presentation transcript:
Preliminary Laboratory Studies of the Photoprocessing of PAH / H 2 O Mixtures in the Interstellar Medium John Thrower Department of Chemistry, School of Engineering and Physical Sciences Heriot Watt University, Riccarton, Edinburgh, EH14 4AS, UK
Polycyclic Aromatic Hydrocarbons (PAHs) Planar aromatic carbon networks May be origin of: Unidentified Infra-Red emission bands (UIRs) Diffuse Interstellar Bands (DIBs) No single PAH has been definitively identified in the ISM Large PAHs may form part of the carbonaceous grain core population Smaller PAHs expected to be present in icy mantles around interstellar grains H 2 O ice dominated environment
PAHs in the Interstellar Medium Some evidence for conversion to more complex organics UV / ion irradiation → Photochemistry Need to understand fundamentals – focus on desorption Simple model of PAH – C 6 H 6 ISO – possible detection of C 6 H 6 C 6 H 6 less stable than larger PAHs Experimentally more convenient UV Spectrum in gas/liquid phases well known Similar in the solid phase Solid state spectra obtained by Open University group at the Daresbury Laboratory Several possible channels following irradiation
The Experiment New Ultrahigh Vacuum (UHV) system at Central Laser Facility Surface Science Techniques LEED, AES, TPD, RAIRS Line of sight mass spectrometry (LoS-MS) Time of flight mass spectrometry (ToF-MS) Model the interstellar dust-grain interaction Nanosecond pulsed lasers Induce desorption / photochemistry in model interstellar ices
Sapphire Substrate Eliminate metal mediated effects Easily cooled to cryogenic temperatures Held at UHV (<2×10 -10 mbar) Substrate base temperature ~60-80 K Closed cycle helium cryostat Ices deposited by introducing gases into chamber via a fine leak valve – each layer = 200 L Irradiate at 248.8 nm (on-resonance), 250.0 nm (near-resonance) and 275.0 nm (off-resonance) Laser powers: “low” (1.1 mJ/pulse) and “high” (1.8 mJ/pulse) Sapphire C6H6C6H6 C6H6C6H6 C6H6C6H6 H2OH2OH2OH2O H2OH2O
The Experiment Dye Laser Nd:YAG QMS MCS trigger Photon induced desorption Time of Flight (TOF) Liquid N 2
Benzene Desorption More desorption “near- resonance” than “on- resonance”! Desorption observed off-resonance from benzene absorption 250.0 nm feature “different” Sapphire C6H6C6H6
Benzene Desorption More desorption at higher energies – expected as photon flux is increased. Suggests single photon process Similar wavelength dependence and peak positions Cannot use 1.8 mJ at 275 nm with current optics Sapphire C6H6C6H6
Benzene UV Absorption Separate UV transmission experiments at Daresbury Laboratory by OU Group Shift in peak position due to phase change between 60 K and 70 K Temperature / K Peak Position / nm 60249.6 70248.8 More absorption at 250.0 nm than 248.8 nm
Water Desorption Noisy due to higher water background Very little water desorption No strong wavelength dependence Water does not absorb at these wavelength – any desorption must be substrate mediated Sapphire H2OH2O
Benzene Desorption from Layered Systems Less benzene desorbed when benzene is underneath water => benzene needs to diffuse through water Other systems similar Slight increase in benzene on water cf. benzene alone?
Water Desorption from Layered Systems More desorption when benzene is present – energy transfer from benzene to water Sharp feature on same timescale as benzene desorption Very slow broad feature when water is on top of benzene – origin?
Maxwell-Boltzmann Fitting Fit to following Maxwell-Boltzmann function: Where: t is the time of flight corrected for time between ionisation and detection in QMS L is the physical distance from sample to point of ionisation T is effective temperature m is molecular mass k is the Boltzmann constant A is a scaling parameter Only single Boltzmann component fitted – may need multiple components.
Maxwell-Boltzmann Fitting Desorption following resonant absorption by benzene produces “hotter” molecules than off- resonance. On-resonance desorption is combination of substrate mediated and resonant effects New non-absorbing substrate? Sapphire C6H6C6H6
Maxwell-Boltzmann Fitting Benzene alone peak at similar temperature at high power Benzene “cooler” when water is present May be a difference in T between the two layering configurations? All give rise to “hot” benzene
Conclusions Benzene and Water desorption strongest on-resonance with benzene absorption at 250 nm Water desorption enhanced by presence of benzene Energy transfer from benzene to water Benzene comes off “hot”, i.e. Boltzmann temperature ~1000 K Astrophysical implications – highly energetic molecules Evidence for “cooler” molecules following substrate mediated desorption (~500 K) Overlayer of water reduces amount of benzene desorbed – but it still has T>900 K
Future Work Eliminate substrate mediated desorption channel Study only pure resonance effects Substrate mediated channel may be relevant though. Absorption by grain may be important. Silicate grain mimics in parallel to meteorite derived material studies Move on to looking at mixtures More realistic representation of interstellar environment PAHs So far only benzene has been studied, PAHs have greater stability Photochemistry Only followed Benzene (78 amu) and water (18 amu) mass numbers with QMS Utilise RAIRS for infrared studies – any evidence for reaction products
Acknowledgements Academic Team Prof. Martin McCoustra (Heriot-Watt) Dr Wendy Brown (UCL) Dr Helen Fraser (Strathclyde) Prof. Nigel Mason (OU) Postdocs Dr Mark Collings (Heriot-Watt) Dr Daren Burke (UCL) Dr Anita Dawes, Dr Phil Holtom, Dr Paul Kendall and others (OU) Students Farah Islam (UCL) Sharon Baillie (Strathclyde Summer Student) Jenny Noble (Strathclyde Summer Student) Any others I’ve missed Laser for Science Facility Dr Ian Clark Dr Sue Tavender Ruth Webster David Workshops at RAL and Nottingham