Thermalization in a nanodroplet: evaporation vs reactivity

Molecular systems under irradiation are present in the living world and inert matter. From a macroscopic point of view, they are made up of a very large number of molecules but the action of radiation acts through the electrons located on a molecule thus creating, locally and on short time scales, a situation clearly far from the thermodynamic equilibrium. Studying molecular nano-systems under irradiation provides access to how the energy deposited in a molecule will be redistributed into the system through interactions between molecules. The velocity distributions of an evaporated molecule measured for protonated methanol nano-droplets have a bimodal behaviour with, as observed for water, 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 following irradiation between molecular evaporation and an elimination reaction will contribute to constrain hypothesis on the formation of prebiotic molecules under interstellar conditions. The results for the water-methanol mixed nano-droplets were compared with those obtained with pyridine doped water nano-droplets and protonated water nano-droplets. 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 pyridinium. 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. (author)