interaction of atomic and molecular hydrogen with amorphous water ice surfaces mimicking interstellar dust.
Interaction de l'hydrogène atomique et moléculaire sur des surface de glace d'eau amorphe simulant les grains de poussière interstellaire
Résumé
The interstellar medium (ISM) is the place surrounding the stars. It is constituted of gas and dust coming from the ejecta of some stars and the explosion of some others. Interstellar dust grains can be carbonaceous or composed of silicates, iron and magnesium. Over 120 molecular and atomic species are detected so far in the ISM. Molecular hydrogen is the most abundant and by far the most important since it is found in three of four molecules essential for life: water (H2O), methane (CH4), ammonia (NH3) and carbon monoxide (CO). The physical-chemistry that leads to the formation of molecules and of stars afterwards can be divided in two: the gas phase chemistry and the gas-surface chemistry. In the extreme conditions (very low both temperature and gas density) that exist in some places of the ISM, gas phase reactions are highly inecient, especially for the formation of molecular hydrogen whose abundance can only be explained by its formation occurring on the surface of dust grains. These grains play the role of catalysts and help evacuating the excess energy released by the molecules formed. This thesis is mainly an experimental contribution to the study of the interaction and formation of molecular hydrogen on water ice surfaces mimicking the ice mantles that cover dust grains in the dark clouds of the ISM. For this purpose, combining ultra-high vacuum techniques, cryogenic systems, atomic and molecular beams, mass spectrometry as well as theoretical modelling, several experiments is conducted using the FORMOLISM (FORmation of MOLecules in the InterStellar Medium) experimental set-up. In this thesis work, the sticking of molecular hydrogen and deuterium is studied in detail and the sticking coecient is found to be highly dependent on the gas temperature and mass. In dark clouds, where grains are covered with ice and Tgrain=Tgas=10 K, the sticking of H2 is found to be 74% and that of D2 82%. Other experiments highlight the mobility of hydrogen atoms on porous water ice at 10 K. This mobility has been questioned for a long time and has never been really proved experimentally. The diusion barrier of the atoms is found to be equal to 22+-2 meV, in agreement with several theoretical calculations. This value is equivalent to a diffusion time of 12 ms from one adsorption site to a neighbour site. A final set of experiments have been conducted in order to study the formation and de-excitation of nascent hydrogen molecules on porous and non-porous amorphous water ice in the conditions of quiescent dark clouds. These experiments have shown that more than 90% of the formation energy is deposited into the ice in the porous case. This energy transfer can be explained by two phenomena: (1) the saturation coverage of the ice surface by H2, and (2) the porosity of the ice that recaptures the nascent molecule helping it to relax. These experiments show that less than 10% of the newly formed molecules are released in the gas phase in an excited ro-vibrational states both in the porous and the non-porous ice cases. These results may explain why the observations of several teams trying to detect excited molecular hydrogen in dark quiescent clouds were unsuccessful.
Parmi les différentes structures de l'univers existe ce qu'on appelle le milieu interstellaire (MIS). C'est un endroit où gaz et poussière co-existent et interagissent en parfaite harmonie. L'hydrogène moléculaire est l'espèce la plus abondante et de loin la plus importante du gaz interstellaire. Elle est à la base de trois sur quatre des molécules les plus essentielles à l'apparition de la vie : l'eau, le méthane, l'amine et le monoxyde de carbone. La physico-chimie du MIS qui mène à la formation de nouvelles molécules est divisée en deux : les réactions en phase gazeuse et les réactions sur les grains de poussière qui s'est révélée la voie de formation la plus efficace pour l'hydrogène moléculaire. Ce travail de thèse est une contribution expérimentale à l'étude de l'interaction et de la formation de l'hydrogène moléculaire sur les surface de glace d'eau amorphe qui couvrent les grains de poussière dans les nuages sombres du MIS. Dans ce but, en réunissant techniques ultravides, systèmes cryogéniques, jets atomiques et moléculaires, spectrométrie de masse et modélisation, plusieurs expériences ont été faites en utilisant le dispositif FORMOLISM (FORmation of MOLecules in the InterStellar Medium).
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