Université de Cergy-Pontoise, FRANCE

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1 Water formation through O 2 +D pathway on cold silicate, graphite and amorphous water surfaces of interstellar interest Henda Chaabouni Marco Minissale, François Dulieu LERMA-LAMAp, UMR 82 du CNRS, Observatoire de Paris Université de Cergy-Pontoise, FRANCE Giulio. Manicò Dipartimento di Fisica ed Astronomia Università degli Studi di Catania, ITALY

2 Acknowledgments Université de Cergy (LAMAp/LERMA Laboratory) Italy, Catania Dipartimento di Fisica ed Astronomia Università degli Studi di Catania Francois Dulieu Marco Minissale Emanuele Congiu Saoud Baouche Jean Louis Lemaire Giulio Manicò (Model) Mario Accolla Valerio Pirronello 2

3 Background Water in the ISM Water is detected in the ISM and outside of our Solar System as Gas, and Ice. T gas = 50 K-00 K T gas = 0 K Gas H 2 O: Gas H 2 O: ice T dust = 0 K T dust =T gas = 0 K

4 Background Previous studies Gas-surface chemistry of water formation in the ISM H + O OH H + O 2 HO 2 H + OH H 2 O H + HO 2 H 2 O 2 Dulieu et al A A (200) Jing et al, APJL (203) H + H 2 O 2 H 2 O + OH H + OH H 2 O Miyauchi et al. Chem.Phys.Lett (2008) Ioppolo et al. APJ (2007), PCCP (200) Thick film of O 2 (0-30 layers) at 0 K H + O 3 O 2 + OH H + O 2 HO 2 H + HO 2 H 2 O 2 H + H 2 O 2 H 2 O +OH H+ OH H 2 O H 2 +OH H 2 O +H Efficient formation of H 2 O and H 2 O 2 in the Multi-layer regimes H 2 O 2 > H 2 O Mokrane et al APJL(2009) Romanzin et al, JCP (20) H + HO 2 OH+OH H 2 O 2 Cuppen et al. PCCP (200)

5 Project O 2 + D in the sub monolayer regime SILICATE GRAPHITE WATER ice Graphite (~00 nm) amorphous Olivine ( Mg.8 Fe 0.2 SiO 4 ) HOPG slab Highly Ordered Pyrolytic Graphite 0-20 ML film Amorphous Solid Water ices (H 2 O vapor deposition)

6 Experiments Qudrupole mass spectrometer QMS (Gas phase detection) FORMOLISM setup (LERMA, Cergy) (MCT) detector QMS TPD Temperature programmed desorption Heating rate: β =0.04 K/s T = 0 K+ βt Surface: 0 K to 220 K DED During Exposure Desorption Surface: 0 K Triply differentially pumped beam-lines 83 UHV chamber 0 - mbar Sample (siliacte, graphite, water) Surface: 0 K Cryocooler 5 K K K FT- IR Spectrometer Bruker Tensor 27 O 2 Micro-wave 2.45 GHz, 200 W D Dissociation D 2 (70%) Reflection Absorption Infra-Red Spectroscopy RAIRS (Insitu Solid phase detection) ( ) cm -

7 RESULT : Water formation on Silicate surface Silicate at 0 K g) f) e) d) c) b) a) n (-OD) RAIRS RAIR specrta (2n6) O 2 exposure dose (ML) (D 2 O + D 2 O 2 ) at 2404 cm D 2 O 2 at 207 cm - ~ ML of D 2 O ice 0 5 molecules.cm Successive deposition of O 2 and D atoms (0.2 ML) O min D-atoms Low surface coverage D 2 O > D 2 O 2 Sub-monolayer regime Chaabouni et al. J. Chem. Phys (202)

8 RESULT 2: Effect of the substrate on O 2 +D D (24 min) D (24 min) D (24 min) ML O 2 Silicate 0 K ML O 2 Porous ASW 0 K ML O 2 non Porous prépared ASW 0 K at 20 K RAIRS, Ts = 0 K D 2 O TPD, Ts = 0 K 220 K 00 % O 2 5 % O 2 5 % O 2 2 % O 2 7 % D 2 O 55 % D 2 O 65 % D 2 O Low formation yield of D 2 O water ice on the Silicate surface at 0K

9 RESULT 2: Effect of the substrate on O 2 +D Low formation yield of D 2 O on Graphite and Silicate surfaces ~ 65 % ~ 2 % ~7 % Dulieu Dulieu et et al, al, Sci.Rep Sci.Rep (203) (203)

10 RESULT 3: Chemical desorption DED QMS D 2, Silicate Surface at 0 K D 2 O gas D O 2 ( ML) Monitoring with the QMS the species desorbing into the gas phase during the exposure of D-atoms on ML (O 2 ) ice at 0 K. D 2 O gas Non thermal desorption of D 2 O upon formation on the surface at 0 K Chemical desorption of D 2 O

11 Reaction routes for water formation Silicate T s =0 K H=-2.4 ev H=-4.9 ev Eley Rideal mechanism -α α H=-3.7 ev D H=-.6 ev D Barrierless reaction D + OD D 2 O OD + OD H= -3.eV Barrier (2000 K) D H= -5.2 ev 2 D D 2 O gaz Chemical desorption of D 2 O by the exothermic reaction D + OD D 2 O H= ev D D 2 O gaz Cuppen et al. PCCP (200) Chaabouni et al. J.Chem.Phys (202) Oba et al APJ (203)

12 Kinetic O 2 +D reaction on Silicates ML (O 2 ) D atoms (0 min) (6 min) (2 min) (24 min) (35 min) TPD QMS T S = 0 K to 220 K (M 32) ML O min D ML ML O +9 min min D ML ML O +2 min D min ML ML O min 2 +24min D ML O min D (M 20) ML ML O 9 min min D ML ML O 2 +2 min D min ML ML O +24min min D ML O min D (M 36) (M 36)

13 Kinetic O 2 +D reaction on Silicate Modeling (TPD) Experiments (MODEL) O 2 + D k k O 2 D O 2 D +D OD + OD = 0,7 k D 2 O 2 - =0,3 D 2 O 2 +D k D 2 O (solid) + OD OD + D e V k H 2 O(g) Cd (gas) D 2 O (solid) Rate constante of reactions without Barrier k = Rate constant of reaction with a Barrier k =0.09 Branching ratio of the reaction α=0,7 Chemical desorption rate of D 2 O Cd= 80 %

14 Conclusions The formation of water through O 2 +D is efficient in the sub-monolayer regime. The formation yield of D 2 O water ice depends on dust grain surfaces. SILICATE and GRAPHITE O 2 + D DO 2 + D OD + D + D 2 O OD + D D 2 O (gas) The heat of the exothermic reaction desorbe water into the gas phase High chemical desorption rate (~ 80 %) WATER ICE D O 2 + D DO 2 + D OD +OD D 2 O 2 D 2 O (solid) + OD WATER ICE dissipate the excess energy released from exothermic reactions Low chemical desorption rate (~ 35 %). Astrophysical implications: The chemical desorption of water has an impact on the gas phase composition of astrophysical environment, and can affect Stars and Planets formation.

15 LERMA CERGY François Dulieu (Directeur), Henda Chaabouni (MCF), Emanuele Congiu (MCF), Saoud Baouche (Ingérieur), Audrey Moudens (MCF), Henri Lemaîre (Doctorant), Marco Minissale (Docteur) Stephan Diana (informaticien), François Lachèvre (mécanicien)