F. Dulieu *, L. Amiaud, S. Baouche, A. Momeni, J.-H. Fillion, J.L. Lemaire
Laboratoire dÕEtude du Rayonnement et de la Matie`re en Astrophysique, UMR 8112 du CNRS, Observatoire de Paris et Universite´ de Cergy Pontoise, Neuville II, 5 mail Gay Lussac, FR-95031 Cergy Pontoise Cedex, France
Received 25 November 2004; in final form 13 January 2005
Abstract
Experimental studies of adsorption and desorption of molecular hydrogen on an amorphous porous solid water ice surface between 10 and 35 K reveal a very efficient isotopic segregation process. A statistical model, which take into account thermodynamic aspects of adsorption sites and isotopic competition, is proposed to understand the enhancement of deuterium fractionation. This mechanism could play a key role in chemistry at the surface of interstellar dust grains.
Ó2005 Elsevier B.V. All rights reserved.
www.elsevier.com/locate/cplett Chemical Physics Letters 404 (2005) 187–191
Cet article est une lettre pr´esentant les premi`eres exp´eriences sur les m´elanges isotopiques et la possibilit´e d’une descritpion du ph´enom`ene `a l’aide de notre mod`ele.
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Interaction of atomic and molecular deuterium on non porous amorphous water ice surface between 8 and 30 K
L. Amiaud, F. Dulieu , J-H. Fillion, A. Momeni, J. L. Lemaire,
LERMA, UMR8112 du CNRS, Observatoire de Paris et Universit´e de Cergy Pontoise, 5 mail Gay Lussac, 95031 Cergy Pontoise Cedex
Abstract :
Molecular and atomic interaction of hydrogen on dust grains covered with ice at low temperature is a key mechanism in dark interstellar cloud. We have experimentally studied the interaction of atomic and molecular deuterium on non porous amorphous water ice surface between 8 and 30 K, in conditions compatible with an extrapolation to astrophysical context. The adsorption energy of D2 presents a large distribution, as already observed on porous water ice surface. Sticking coefficient of D2 increases linearly with the number of already adsorbed deuterium molecules on the surface. Formation occurs via a prompt reaction that releases molecules in gas phase. At least a part of the newly formed molecules are in a vibrationnaly excited state (v = 1 - 7). The formation efficiency increases with the presence of already adsorbed molecules, probably because the sticking coefficient of the atoms also increases. We have measured the formation efficiency of the molecules in the presence of absorbed molecules as it is expected to occur in the interstellar medium. The formation efficiency decreases rapidly with the temperature and is already zero at 13 K. This allows to estimate an upper value limit of the atom adsorption energy Ea 33 meV, in agreement with previous calculations.
Soumis
R´esum´e
La physico-chimie et l’´evolution des diff´erents milieux qui constituent le milieu inter-stellaire d´ependent ´etroitement de H2, son principal constituant mol´eculaire. En parti-culier, la connaissance incompl`ete du bilan ´energ´etique et de l’efficacit´e de la r´eaction de formation d’hydrog`ene mol´eculaire par catalyse h´et´erog`ene sur les grains de poussi`ere est une source importante d’incertitude dans la description de la dynamique du milieu, no-tamment lors de la formation d’´etoiles. L’´etude de cette r´eaction et de ses sous-processus (collage et diffusion sur les grains, d´esorption) est abord´ee th´eoriquement et exp´ erimen-talement depuis plus de 40 ans.
Cette th`ese vise par une approche exp´erimentale `a caract´eriser la r´eaction de for-mation d’hydrog`ene mol´eculaire `a la surface des glaces d’eau. Elle s’articule autour du dispositif FORMOLISM. Ultravide, cryog´enie, jets atomiques, spectrom´etrie de masse et spectroscopie UV sont r´eunis pour ´etudier en particulier les effets de l’h´et´erog´en´eit´e et de la porosit´e de la surface. L’´etude de la d´esorption de l’hydrog`ene mol´eculaire s’est r´ev´el´ee indispensable `a l’interpr´etation des exp´eriences de formation. Nous avons me-sur´e les distributions d’´energies d’adsorption de H2, HD et D2. Ces mesures permettent d’estimer la quantit´e d’hydrog`ene mol´eculaire en surface des grains interstellaires. La pr´esence d’hydrog`ene mol´eculaire modifie l’efficacit´e de la r´eaction. Un m´ecanisme de s´egr´egation isotopique a ´et´e mis en ´evidence et son importance pour la deut´eration de l’hydrog`ene mol´eculaire en surface des manteaux de glace a ´et´e ´etudi´ee. Les exp´eriences sur la formation r´ev`elent que sur les glaces poreuses l’´energie d´egag´ee par la r´eaction est transmise `a la surface par la r´etention des mol´ecules form´ees. La r´eaction reste efficace `
a des temp´eratures plus ´elev´ees (20 K) que sur les glaces non poreuses (13 K). Sur ces derni`eres, les mol´ecules form´ees sont directement lib´er´ees en phase gazeuse o`u elles sont d´etect´ees dans des ´etats rovibrationnellement excit´es.
Abstract
The molecular hydrogen, a major component of the interstellar medium (ISM) is formed on dust grains through surface catalysis. This key reaction is a source of uncer-tainty in the description of the dynamic of the ISM, as well for the reaction efficiency and for its energy budget. This reaction and its sub-processes (sticking and diffusion on the grain surface, desorption) are studied here experimentally in the case of molecular hydrogen formation on water ice surfaces similar to those covering dust in the cold dense ISM.
Ultra-high vacuum, cryogenics, atomic beams, mass spectrometry and UV spectro-scopy are combined in the experimental setup named FORMOLISM. We have studied the effects of surface heterogeneity and porosity on the reaction. We have focused on molecular hydrogen desorption to improve the interpretation of formation experiments and because the molecular hydrogen adsorbed on grains modify the formation efficiency. Adsorption energies of H2, HD and D2 on these ices are obtained. A mechanism of isoto-pic segregation is highlighted and the deuteration of molecular hydrogen on ice mantles is quantified. The study of molecular formation reveals that on porous ices the reac-tion energy is released to the surface because of the efficient recapture of the formed molecules, and that the reaction is effective up to temperatures higher (20 K) than for nonporous ices (13 K). For these latter, the formed molecules are directly released into the gas phase where they are detected in rovibrationally excited states.