1 / 22

Ion Irradiation of Astrophysical Ice Analogues

4 th Annual LEIF Meeting, Belfast. Ion Irradiation of Astrophysical Ice Analogues. Anita Dawes. Introduction. Not much is known about the mechanisms involved in solid state chemistry in astrophysical environments. Over 120 molecular species have been detected in space

lowell
Download Presentation

Ion Irradiation of Astrophysical Ice Analogues

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 4th Annual LEIF Meeting, Belfast Ion Irradiation of Astrophysical Ice Analogues Anita Dawes A.Dawes@ucl.ac.uk

  2. Introduction • Not much is known about the mechanisms involved in solid state chemistry in astrophysical environments. • Over 120 molecular species have been detected in space • Abundances and formation cannot be explained by gas phase chemistry A.Dawes@ucl.ac.uk

  3. >120 Interstellar and Circumstellar Molecules Glycine ? Acetic Acid Benzene Glycolaldehyde Cyanopolyynes Formic Acid http://www.cv.nrao.edu/~awootten/allmols.html National Radio Astronomy Observatory A.Dawes@ucl.ac.uk

  4. Ices in Space... Whittet et.al.A&A 315, L357-L360 (1996) Gibb et.al. ApJ 536, 347-356 (2000) W33A Gibb et.al. ApJ 536, 347-356 (2000) A.Dawes@ucl.ac.uk

  5. Ices in the outer Solar System A.Dawes@ucl.ac.uk

  6. Introduction • Not much is known about the mechanisms involved in solid state chemistry in astrophysical environments. • Over 120 molecular species have been detected in space • Abundances and formation cannot be explained by gas phase chemistry • Require laboratory data to understand the mechanisms involved in condensed phase molecular formation/destruction. • The nature of ices and their processing depends on the environment in which they are found... A.Dawes@ucl.ac.uk

  7. Radiation Environments • The astrophysical ices can be broadly divided into 3 environments: • Icy grain mantles in the ISM • Dense clouds: Lyman-a, cosmic rays • Cold diffuse clouds: UV dominated • Cometary ices • Oort cloud: cosmic ray dominated • Icy solar system bodies • e.g. Mars polar caps: solar wind, solar UV • Galilean satellites: magnetospheric ions (dominant) & solar wind – O+, O2+, O3+, O4+, O6+, S+, S2+, S3+, S4+, S5+, S2+, SO2+, Na+, K2+, C6+, H2O+, H3O+, OH+, H+, He+, H2+ and H3+ • The ices can be physically characterised by the: • Thickness, temperature & composition • Energy, flux & type of processing radiation • In our laboratory regime: • Ices are already present, i.e. not concerned with ice accretion / formation •  thick ice layers (to ignore the effect of the substrate) • Ion, photon and electron irradiation A.Dawes@ucl.ac.uk

  8. 50.15 Transmission Wavelength Portable Apparatus Temperature controller Cryogen inlet via transfer line • HV (UHV) chamber: • P~10-7-10-10 mbar • CaF2 substrate for transmission spectroscopy • 120 nm – 10 mm • Temperature: • LN2 / LHe cryostat • >30 K • Rh-Fe sensor • Resistive coax. Heater • 4 ports • Sample deposition • Spectroscopy • Irradiation • Transmission spectra recorded vs. wavelength / frequency Liquid nitrogen exhaust Electrical feed-through Rotary feed-through Liquid Helium / Liquid Nitrogen Cryostat Ion gauge To pumping station Resistive heater Rhodium-iron RTD CaF2 substrate Copper sample mount Detectors (Spectroscopy): UV-VIS / FTIR spectrometer PMT Sources (Spectroscopy): UV-VIS / FTIR spectrometer Synchrotron A.Dawes@ucl.ac.uk

  9. To rotary Feed-through Turbo Pump Pirani Gauge To Rotary Pump for roughing Baratron Precision Leak Valve Gas regulator Gas reservoir ON/OFF Valve Liquid sample UHV Gas from Lecture bottle ON/OFF valve Needle valve Sample Preparation • Ice layers are vapour deposited directly onto a cold substrate. • The gases are prepared in a reservoir prior to dosing • Pure gases or mixtures • Sample thickness is determined from the pressure of gas deposited. • Ice thickness is calibrated by measuring the absorption through the sample (column densities) or analysing interference fringes ~10m thick CO2 ice (left) ~3 m thick H2O ice (right) A.Dawes@ucl.ac.uk

  10. Sample Irradiation • Once deposited, the samples are irradiated with either photons, ions or electrons • The products may be probed at regular intervals by spectroscopy • UV-VIS & VUV : Electronic Structure • FTIR : Vibrational Structure 1 hour of irradiation in the lab is equivalent to 1000 years irradiation in space! A.Dawes@ucl.ac.uk

  11. What are we currently looking at? • Study of H2O, CO2 and H2O:CO2 ices • These are two of the most abundant molecules • Present in all astrophysical environments (grain mantles, comets, Galilean satellites, Mars & Triton. • Irradiation with ions • 100 keV H+ • Low energy (1-5 keV) singly charged ions • Low energy (1-5*q keV) multiply charged ions • implantation – reactive ions: C+ on H2O and H+ on CO2 • Irradiation with photons • Zero order Synchrotron radiation • Discrete wavelengths (synchrotron grating monochromator) • Products we are looking for in H2O:CO2 ices: • Carbonic acid (H2CO3) • CO, CO3, H2O:CO2 complex, HCO, O3(?) and others (?) • FTIR spectra A.Dawes@ucl.ac.uk

  12. Irradiation of H2O:CO2 ice at 90 K with100 keV H+ After 1 hour irradiation CO2 CO2 CO2 CO2 13CO2 13CO2 H2O H2O H2CO3 (1488) H2CO3 (1703) H2CO3 (1295) CO (2140) H2CO3 (2850) CO3 (2044) H2O:CO2 H2O:CO2 H2CO3 (2580) H2O H2O H2O H2O A.Dawes@ucl.ac.uk

  13. Irradiation of H2O:CO2 ice at 50 K with5 keV H+ After 0.5 hour irradiation CO2 CO2 H2O H2O H2CO3 (1703) 13CO2 13CO2 H2CO3 (1488) H2CO3 (1295) NO CO3! H2CO3 (2850) CO (2140) H2O:CO2 H2O:CO2 H2CO3 (2580) H2O H2O H2O H2O A.Dawes@ucl.ac.uk

  14. Crystalline H2O CO2 H2O CO2 H2O H2O H2O CO CO2 H2O CO2 H2O H2O H2O Warm-up after H+ Irradiation of H2O:CO2 ice CO2 H2O H2O 220 K 50 K 100 K 120 K 180 K 160 K 250 K 200 K 140 K Temp: CO2 H2O H2O H2CO3 CO H2CO3 H2O A.Dawes@ucl.ac.uk

  15. H2CO3 yield after irradiation withH+,He+,O+andNe+(all at 5 keV) Yield depends on: • Ion range? • Reactive, unreactive ion? Low or no yield with multiply charged ions (N3+, N5+, N6+) – not shown here: • Lack of secondary electrons? • Small penetration depth? A.Dawes@ucl.ac.uk

  16. The CO profile The 2152 cm-1 CO feature possible origin: • formation at different sites in the ice matrix (substitutional / interstitial) (Sandford et.al. ApJ 329, 498-510, 1998) • CO diffusion into unirradiated ice and interaction with the dangling OH bonds (Palumbo, J Phys Chem A, 101, 4298-4301, 1997) (Sandford et.al. ApJ 329, 498-510, 1998) A.Dawes@ucl.ac.uk

  17. CO formation by irradiation of H2O:CO2=1 with different ions (5*q keV) at 50K The 2152 cm-1 CO feature : • Increases with mass of ion  interstitial ? • Increases with decreasing penetration depth of ion  diffusion ? • Repeated experiment at ~ 100 K with heavier ions.....No 2152 feature!  diffusion ? (CO partially sublimes >27 K) CO Ne+ O+ N3+ He+ H+ A.Dawes@ucl.ac.uk

  18. Irradiation of pure H2O with 2 keV C+ Polar Component Apolar Component CO 2152 cm-1 feature 00:00 00:15 00:30 00:45 01:00 01:15 01:15 Irradiation time A.Dawes@ucl.ac.uk

  19. Summary of Results... More questions...? • CO, CO3 and H2CO3 were seen after irradiation of H2O:CO2 with 100 keV H+ But... • No H2CO3 seen with highly charged ions (or signal too weak?) • Low penetration depth? • Lack of secondary electrons? • PE vs KE effect? • No CO3 seen after irradiation of H2O:CO3 with low (<5*q keV) energy/multiply charged ions! • KE vs PE effect? • Nuclear vs electronic stopping? • 2152 cm-1 feature of CO was seen in H2O:CO2 irradiated ice with heavier ions at T<60 K But... • 2152 cm-1 feature was not seen in H2O:CO2 irradiated ice with any ion at T > 90 K Many more... A.Dawes@ucl.ac.uk

  20. To do... Search for Answers! Changing the sample... • Temperature • Diffusion (e.g. CO) • Activation energy • Crystalline vs. amorphous ice • Get down to 10K!! • Composition • Isotopic substitution to identify reaction pathways • Ratio of components • Structure • Porosity • Crystalline vs. amorphous ice • Thickness • Ion / photon penetration depth • Implantation Changing the beam... • Singly vs. multiply charged ions • kinetic vs. potential effects? • Secondary electrons... • Different energies • nuclear vs. electronic stopping effect of ions (latter dominates as energy increases) • Different ions • Effect of ion mass/momentum/velocity • Reactive / unreactive ions • Implantation – chemical vs. physical effects • Low energy electron irradiation • Secondary electron effect following ion irradiation • Ion vs. Photon irradiation • Systematic comparison A.Dawes@ucl.ac.uk

  21. GeneralSummary • More experimental work is needed to fully understand the mechanisms involved in synthesis of molecules under astrophysical environments... • The design of the new apparatus allows flexibility to perform a wide variety of experiments, using different sources to irradiate samples and implement a variety of spectroscopic techniques using different instruments. • Ion accelerators – Belfast (singly & multiply charged ions) • Synchrotron Sources – Århus & Daresbury (irradiation & spectroscopy... Circular dichroism) • Electron and photon sources • These experiments will enable us to better understand the processes behind chemical processing if ices in the ISM and the planets, satellites, comets and meteorites within our solar system.(The apparatus can also be adapted to study atmospheric ices e.g. polar stratospheric clouds) • A systematic experimental approach is required to identify reaction pathways and intermediates whilst pinning down the different variables (both sample and irradiation parameters). • Many more experiments to come... A.Dawes@ucl.ac.uk

  22. Acknowledgements Bob McCullough and Ian Williams (QUB) Roland Trassl (Geissen University) Søren Vrønning Hoffmann (ASTRID, Århus) Nigel Mason Stephen Brotton Mike Davis Philip Holtom A.Dawes@ucl.ac.uk

More Related