Oxidation processes on cold surfaces, diffusion and reactivity

Transcription

1 Marco Minissale Université de Cergy-Pontoise and Observatoire de Paris Oxidation processes on cold surfaces, diffusion and reactivity Leiden, 13th March 2013

2 What we know about oxygenation? Astrophysical interest M a s s O O2 O3 H Yes (H2O) Yes (H2O) Yes (H2O) N O Nee!? Yes (O2) Yes (O3) Maybe (2 O2) CO Yes (CO2) Nee Nee NO Yes (NO2-NOy) Yes (NO2-NOy) Yes (NO2-NOy) O2 Yes (O3) Nee Nee CO2 Nee Nee Nee NO2 Yes - - O3 Maybe Nee Nee

3 SET UP: FORMOLISM LERMA-LAMAp Université de Cergy-Pontoise

4 Main Chamber Pression= mbar corresponding to a molecular/atomic density of about 2*106 cm-3. Leak valve equipped with a microchannel doser. When it is opened, water, contained in a small vial, can diffuse into the chamber via a micro capillary array. Sample holder (SiO)x,Graphite 6.5 K-350 K

5 Main Chamber Pression= mbar corresponding to a molecular/atomic density of about 2*106 cm-3. Leak valve equipped with a microchannel doser. Gibb et al, 2000 When it is opened, water, contained in a small vial, can diffuse into the chamber via a micro capillary array. Sample holder (SiO)x,Graphite 6.5 K-350 K

6 Beamlines Each beamline has three differentially pumped (atomic/molecular) stages to the MB 1 mbar Chamber mbar Microwave cavity for dissociating molecular H2, D2, O2,N2 etc Surfatron 2.45 GHz Chamber 2 Chamber mbar 10-8 mbar Flag

7 Plasma Dissociation rate depends on the flux and the RF power Microwave cavity for dissociating molecular H2, D2, O2,N2 etc

8 Experimental Methods RAIRS (Reflection Adsorbtion InfraRed Spectrocopy) Mass Spectroscopy through a Quadrupole Mass Spectrometer (QMS) REMPI (Resonance Enhanced Multiphoton Ionization) through a laser

9 What we know about oxygenation? Astrophysical interest M a s s O O2 O3 H Yes (H2O) Yes (H2O) Yes (H2O) N O Nee!? Yes (O2) Nee Yes (O3) Nee Maybe (2 O2) CO Yes (CO2) Nee Nee NO Yes (NO2-NOy) Yes (NO2-NOy) Yes (NO2-NOy) O2 Yes (O3) Nee Nee CO2 Nee Nee Nee NO2 Yes Nee (?) Nee (?) O3 Maybe Nee Nee

10 How to study O Oxygenation O atoms (and O2 molecules) (300 K) irradiate the cold sample We have varied the following parameters: 1) Substrate morphology (ASW, porous and compact, crystalline ice, (SiO)x, graphite) 2) Coverage, from 0.1 to 1 ML 3) Substrate temperature, from 6.5 to 25 K 4) Dissociation rate (from 35% to 75 %)

11 First evidence: O3 formation TPD after deposition of 0.8 ML of O+O2 on silicate τ=60% Minissale et al, submitted TPD after different depositions of O+O2 on ASW τ=74%

12 Reflection-Absorption Infrared Spectroscopy of O3 0.3 ML of O+O2 at 6.5 K 1043 cm-1 No evolution with the time Minissale et al, in preparation

13 Reflection-Absorption Infrared Spectroscopy of O3 No evolution with the increase of surface temperature All the O atoms are consumed at the deposition temperature Area under the peaks Minissale et al, in preparation

14 How can we explain our results? O+O O2 O+O2 O3 When? At deposition temperature (TS) (IR spectrum) How? Eley-Rideal and LangmuirHinsherlwood (diffusion)

15 O3/O2 function of Tsurf 0.3 ML of O+O2 O3/O2 ratio increases with surface temperature Minissale et al, in preparation

16 With Without or Diffusion or not diffusion, what is the difference? O2 < O 3 O2 > O 3

17 k=0 No diffusion O3 O2 We are here O Diffusion is important at low coverage k=100 O3 O2 O Diffusion

18 Ts dependent - surface independent O3/O2 ratio as function of the surface temperature

19 Thermal hopping or Tunneling? k exp(-u0 /T) k exp(-u0 /T)/T Messiah, 1961 Cazaux&Tielens, 2004

20 Diffusion coefficient of O atoms CO diffusion Diffusion constant of O atoms Arrhenius type function Minissale et al, submitted

21 Diffusion coefficient of O atoms E Height Ea a Width Minissale et al, submitted

22 Summary Ediff_Oxygen 400K 400K in Tielens & Hagen (1982), 900K in Cazaux et al. (2010). O+O and O2+O barrier < 190 K Minissale et al. submitted

23 O diffusion - rate constant a delicate balance on subml regime k1 ~ k2 > k3 (>> k4 ~ k5 ) O3 >> CO2 k1 ~ k2 ~ k3 > k4 ~ k5 O3 ~ NO2

24 First application: CO2 formation Nothing on the bottle (Just to be sure) CO+O2 CO+O2 CO+O3 CO+O3 Minissale et al, submitted

25 First application: CO+O On graphite and on ASW CO+O CO2 ΔH=-532 kj/mol Very tiny amount of CO2 formed 0.5 ML of CO+ 0.5 ML of O Minissale et al, submitted

26 First application: CO+O What happens if we change the surface temperature? Diffusion O3 formation O2 desorption CO2 formation Minissale et al, submitted

27 First application: CO+O Diffusion plays the main role for the CO2 formation Reaction barrier of about 600 K (51 mev) Higher than Roser et al 2001 (290 K) And Raut&Baragiola 2011 (? 25 mev) Minissale et al, submitted

28 Second Application: NO2 formation 2NO+O2 2NO2 Competition with NO+NO (NO)2 O2 on NO NO on O2 Codep Minissale et al, 2013 in press CPL

29 Second Application: NO2 formation NO+O3 NO2+O2 2NO+O2 2NO2 Minissale et al, in preparation

30 Second Application: NO2 formation NO+O NO2 2NO+O2 2NO2 NO+O3 NO2+O2 Fast diffusion of O does not prevent the formation of NO2

31 Take home messages O chemistry (because of diffusion) competitive with H chemistry in some interstellar environments (dense clouds). CO+O works but O diffusion (together with high barrier) scants the CO2 formation. NO2 is formed very easily, how is it destructed?

32 My Co-workers Université de Cergy-Pontoise Francois Dulieu, Emanuele Congiu, Saoud Baouche, Henda Chaabouni Audrey Moudens Università di Catania Valerio Pirronello Giulio Manico, Mario Accolla Kapteyn Astronomical Institute Stephanie Cazaux

33 Thank you

34 Atomic beam τ= (S off S n ) (S on S n) ( S off S n ) Rate of Dissociation (S off S n ) ( S on S n ) τ= (S off S n ) Atoms and Molecules in the ground state

35 No excitation colonne 2 colonne 3 colonne ligne 2 ligne 3 ligne 4 ligne

36 Surface reaction mechanisms Eley Rideal Langmuir Hinshelwood (Diffusion)

37 Water ice films on sample holder SPRAY DEPOSITION Compact ice Direct method to grow the ASW quite fast BACKGROUND DEPOSITION Porous ice Water vapor fills uniformily the entire volume of the chamber before its condensation on the cold surface

38

39 What happens with the raise of temperature? Amorphous Crystalline solid water Cubic Hexagonal water

40 Mass Spectroscopy Temperature Programmed Desorption (TPD o TDS) Desorption process the rupture of adsorption bonds and the resulting removal of adsorbed particles from the surface T des E ads Adsorption sites