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M. Dans-ce-chapitre, nous allons exposer les résultats que nous avons obtenus lors de l'étude de la formation de la molécule H 2 sur un grain de poussière interstellaire en considérant le mécanisme Langmuir-Hinshelwood (LH)

. Dans-le-chapitre-précèdent, nous avons postulé que les atomes d'hydrogène ne peuvent être que chimisorbés sur le grain, ce qui ne conduit qu'au mécanisme Eley-Rideal. Que se passe-t-il, si on envisage de physisorber des atomes d'hydrogène sur le grain ? Lorsqu'un atome H est physisorbé, il peut migrer quasi librement sur la surface. Dans ce cas trois mécanismes peuvent être envisagés : 1. le premier type de mécanisme Langmuir-Hinschelwood

. Un-autre-type-de-mécanisme-eley-rideal, où la réaction a lieu entre un atome d'hydrogène physisorbé sur la surface et un atome d'hydrogène provenant de la phase gaz, ER2)

. Dans-ce-chapitre, nous nous intéressons au premier type de mécanisme LH Ce choix est motivé par les raisons suivantes : 1. ce mécanisme LH est sans doute plus efficace que le mécanisme ER2, puisque dans le mécanisme LH les atomes d'hydrogène évoluent dans un monde à deux dimensions

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. Nous-avons-Étudié-le-mécanisme-langmuir, Hinshelwood lorsque les deux atomes d'hydrogène sont physisorbés sur la surface. Or il existe un autre type de mécanisme Langmuir- Hinshelwood où un atome est chimisorbé et un autre est physisorbé sur la surface

. Nous-avons-Étudié-le-mécanisme and . Eley, Rideal lorsque qu'un atome est chimisorbé sur la surface. Toutefois, il est probablement difficile de chimisorber un atome d'hydrogène puisqu'il existe une barrière à la chimisorption considérable par rapport aux conditions énergétiques régnant dans le milieu interstellaire. Notre question est : comment l'atome d'hydrogène se chimisorbe-t-il sur la surface ?