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Dynamique de la formation d'hydrogène moléculaire sur une poussière interstellaire

Abstract : In the interstellar medium (ISM), the formation of H_2 molecules is catalyzed by a dust grain: H+H+grain -> grain+ H_2 Two mechanisms can account for this reaction: the Eley-Rideal (ER) mechanism and the Langmuir-Hinshelwood (LH) mechanism. The wavepacket method has been used to study these two mechanisms at the low temperatures of the ISM. The numerical effort has been reduced by the use of an L shaped grid and of the mapping technique applied to multidimensional grids. The energy collision range studied lies between 0.4meV and 46meV for the ER mechanism and between 4meV and 50meV for the LH mechanism. The ER mechanism has been investigated in collinear geometry on a graphite surface (0001), one of the C atoms of the grain being allowed to move. Taking into account this surface relaxation favors the reaction. Nevertheless, the efficiency of the ER mechanism in ISM condition is poor, because of a small barrier in the entrance valley. The LH mechanism has been studied in full dimensionality on a flat and rigid surface. The main result is that this mechanism is very efficient: in typical ISM conditions, the time for an H atom to diffuse on the grain, to collide another H atom, and to desorb in an H_2 molecule is smaller than the time between two H-grain collision. The reaction does not induce any noticeable effect on the ortho-H_2 / para-H_2 ratio. Now, just like for the ER mechanism, LH reaction yields a huge amount of vibrational excitation.
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Submitted on : Friday, February 18, 2005 - 11:27:05 AM
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Sabine Morisset. Dynamique de la formation d'hydrogène moléculaire sur une poussière interstellaire. Catalyse. Université Paris Sud - Paris XI, 2004. Français. ⟨tel-00008537⟩

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