Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University Electronic Excitation Transport in Ices: A Key Role for Hydrogen Bonding Martin McCoustra John Thrower and Demian Marchione

Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University NGC 3603 W. 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

Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University The Chemically-controlled Cosmos CH 4 Icy Mantle Silicate or Carbonaceous Core nm CO N2N2 H2OH2O NH 3 Heat Input Thermal Desorption UV Light Input Photodesorption Cosmic Ray Input Sputtering and Electron- stimulated Desorption CH 3 OH CO 2 CH 3 NH 2

Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University Processing of icy grains by cosmic radiation (high- energy charged particles) is a crucial process for increasing the chemical complexity of the Universe… but the surface and solid state physics and chemistry of these ices are poorly understood. This especially true of the competition between ice desorption and chemical transformation. The Chemically-controlled Cosmos

Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University C. J. Shen, J. M. Greenberg, W. A. Schutte, and E. F. van Dishoeck, Astron. Astrophys, 2004, 415, 203  Cosmic rays are predominantly protons  The distribution peaks at an energy of around 100 MeV  Proton interaction with the interstellar gas produces Lyman α radiation; each proton producing many photons  Proton interactions with ice produce a distribution of secondary electrons in ice that peaks in the 100 to 500 eV range; each proton producing many electrons The Chemically-controlled Cosmos

Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University Evidence for non-thermal processes in the cold, dense interstellar medium is found in observations of such environments and can be driven by cosmic ray generated VUV photons and secondary electrons. The key question is which is more important! The Chemically-controlled Cosmos

Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University J. D. Thrower, M. P. Collings, F. J. M. Rutten, and M. R. S. McCoustra, Chem. Phys. Lett., 2011, 505,  We have previously reported efficient electron-promoted desorption of benzene (C 6 H 6 ) from a layer of amorphous solid water (ASW)  Water is the predominant component of interstellar ice  C 6 H 6 is the prototypical aromatic hydrocarbon and such species represent the major sink for galactic carbon  Zero order TPD of C 6 H 6 at all sub-monolayer exposures suggests island film growth with some isolated C 6 H 6 between the islands  Efficient process is associated with isolated C 6 H 6 ! Physics versus Chemistry on Ices

Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University  Small reduction in C 6 H 6 ring breathing frequency is consistent with donation of  electron density to a electrophilic centre  C 6 H 6 interacts with the water surface via a weak  hydrogen bond Physics versus Chemistry on Ices

Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University  Desorption of isolated C 6 H 6 has a cross-section of ca. 2  cm 2 in this range cf. 5  cm 2 for H 2 O  Desorption from the C 6 H 6 islands and bulk C 6 H 6 has a cross-section of 5  cm 2  We see no evidence for any chemical transformations only desorption Physics versus Chemistry on Ices J. D. Thrower, M. P. Collings, F. J. M. Rutten, and M. R. S. McCoustra, Chem. Phys. Lett., 2011, 505,

Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University  Water ice supports long-lived excitations at an energy of around eV; each 100 eV electron can produce 8 – 10 excitations  Excitations originate from states rich in O character  Similar states exist in methanol (CH 3 OH) and dimethyl ether (CH 3 OCH 3 )  Repeating our electron- promoted desorption studies on these substrates will tell us if hydrogen bonding is important Exciton Transport via Hydrogen Bonds… G. A. Kimmel, T. M. Orlando, C. Vézina, and L. Sanche, J. Chem. Phys., 11994, 101,

Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University  But first…  Does C 6 H 6 behave on CH 3 OH and (CH 3 CH 2 ) 2 O (diethyl ether as our based temperature is restricted to 110 K and dimethyl ether will not condense) as it does water?  RAIRS shows the interactions are weaker than that of C 6 H 6 and H 2 O  TPD suggests C 6 H 6 / behaves on CH 3 OH as it does on H 2 O but (CH 3 CH 2 ) 2 O wets C 6 H 6 Exciton Transport via Hydrogen Bonds…

Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University Exciton Transport via Hydrogen Bonds…  But first…  Does C 6 H 6 behave on CH 3 OH and (CH 3 CH 2 ) 2 O (diethyl ether as our based temperature is restricted to 110 K and dimethyl ether will not condense) as it does water?  RAIRS shows the interactions are weaker than that of C 6 H 6 and H 2 O  TPD suggests C 6 H 6 / behaves on CH 3 OH as it does on H 2 O but (CH 3 CH 2 ) 2 O wets C 6 H 6

Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University  Hint of a fast desorption from CH 3 OH (red)  Linear hydrogen bonded chains  CH 3 OH has no dangling OH groups at the surface so CH 3 OH must re-orientate on surface if C 6 H 6 is to π hydrogen bond to the surface  No evidence for fast process on (CH 3 CH 2 ) 2 O  No intermolecular hydrogen bonding  C 6 H 6 interacts with the (CH 3 CH 2 ) 2 O via van der Waals forces Exciton Transport via Hydrogen Bonds…

Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University  Hydrogen bonding is crucial  For transporting excitation to the interface  Providing a dissociation coordinate between the bulk hydrogen bonded network and the terminal hydrogen bonded group (C 6 H 6 in this instance)  Key question remains as to the mechanism of the excitation transport  Resonant Energy Transfer cf. Förster  Excited State Proton Transfer cf. Dexter Exciton Transport via Hydrogen Bonds…

Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University  Looking at H 2 evolution in each system under electron- irradiation  Predominantly see H 2 from the substrate and only a hint in the H 2 O system of H 2 from fast desorbing C 6 H 6  More H 2 released from the C- centred species  Is this hinting at different behaviours for O and C centres, especially in hydrogen bonding networks? A Hint of Interesting Things to Come…

Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University  Electron-promoted desorption (EPD; σ typically – cm 2 ) is more efficient than VUV photon-stimulated desorption (PSD; σ typically – cm 2 ) in cold, dense environments and could account for observations of molecules in such regions  Fast EPD may slow accumulation of species hydrogen bonding to H 2 O surfaces especially CO in turn causing segregation of CO on to the grain surface and delaying formation of complex organics on the H 2 O surface Astronomical Impact…

Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University Dr. Mark Collings Dr. Jerome Lasne Vicky Frankland, Rui Chen, John Dever, Simon Green, John Thrower, Ali Abdulgalil, Demian Marchione, Alex Rosu-Finsen and Skandar Taj ££ Framework 7 EPSRC and STFC Leverhulme Trust University of Nottingham Heriot-Watt University ££ Acknowledgements This research was (in part) funded by the LASSIE Initial Training Network, which is supported by the European Commission's 7th Framework Programme under Grant Agreement No