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Thermalisation dans une nanogoutte : évaporation versus réactivité

Abstract : Molecular systems under irradiation are present in the living as well as in inert matter. From a macroscopic point of view, the matter is made up of a very large number of molecules but the action of radiation acts through the electrons located on a molecule and thus, creating locally and on short time scales a situation clearly far from the thermodynamic equilibrium. Studying molecular nanosystems under irradiation provides access to understanding of how the energy deposited in a molecule will be redistributed into the system through interactions between surrounding molecules.The velocity distributions of evaporated molecules measured for irradiated protonated methanol nanodroplets have a bimodal behaviour, as observed for water, including evaporation of molecules with much higher velocities than expected after complete redistribution of energy. In addition, a reaction in the cluster leading to the formation of protonated dimethyl-ether with elimination of a water molecule was observed. The possibility of studying the competition between molecular evaporation and an elimination reaction following irradiation of a nanodroplet will contribute to constrain the hypothesis on the formation of prebiotic molecules under interstellar conditions. The results for the water-methanol mixed nanodroplets were compared with those obtained with pyridine doped water nanodroplets and protonated water nanodroplets. Analysis of the low velocity part of the velocity distributions of the evaporated water molecules shows that evaporation occurs before the complete redistribution of energy in the cluster. It appears that there is less energy available for evaporation of a water molecule when the initial excitation is located on the protonated methanol ion or on the pyrimidium. Thus, unlike the hydronium ion which is fully solvated, impurities promote the growth of these small water clusters, whose presence in the atmosphere facilitates the early stages of aerosol formation
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Thibaud Salbaing. Thermalisation dans une nanogoutte : évaporation versus réactivité. Mécanique des fluides [physics.class-ph]. Université de Lyon, 2019. Français. ⟨NNT : 2019LYSE1163⟩. ⟨tel-02435229⟩

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