interaction of atomic and molecular hydrogen with amorphous water ice surfaces mimicking interstellar dust.

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.