Experimental study of desorption and reactivity of nitrogen bearing species on interstellar dust grains

Abstract : The interstellar medium is a key component of galaxies which is the matrixof the formation of stars and planets. It is composed of interstellar gas and dust grains. The presence of complex organic molecules (COMs) can be understoodvia reactions in the gas phase. However, the synthesis of some key species (i.e H2, H2O, CO2, etc) needs the intervention of the solid-state reactions on dust grains surface. More than 200 molecules including COMs have been detected in ISMand circumstellar shells. Some of these COMs already contain carbon, oxygen,and nitrogen, which are the main chemical elements in the composition of themolecules on which terrestrial life is built. The aim of this thesis is to understand physical and chemical processes (i.e diffusion, desorption, and reactivity) occurring on interstellar dust grains. More specifically, my thesis focuses on the investigation of desorption and reactivity of nitrogen bearing species that lead to COMs by hydrogenation related processes.All experiments were performed with the VENUS set-up located at the LERMACergy laboratory in the university of Cergy Pontoise, France. VENUS is composedof an Ultra High Vacuum (UHV) chamber, so called the main chamber, with abasis pressure around of 10 −10 mbar. Atoms/molecules were injected onto a sample holder in the UHV chamber through a separated four beamline system. The sample holder is controlled in temperature from 7 K to 400 K. Products are probed by using mass spectrometer and reflection adsorption infrared spectroscopy. Different solid states environments have been used: gold surface or water ice substrates (compact amorphous solid water, porous amorphous solid water, and crystalline ice) to mimic different the astrophysical environments.We proceeded experiments involving some atoms and molecules such as hydrogen,deuterium, oxygen, CO, N2, NO, H2O, H2CO, CH3CN, and CH3NC. These specieswere deposited on the cold surface held at 10 K (up to 40 K). CO and N2 weredeposited on water ice substrates to study different adsorbed behaviours as well as the distribution binding energy and the segregation mechanism. We studied theefficiency of atoms (i.e oxygen and hydrogen) penetration into porous water ice.We used NO as a chemical tracer for the penetration of O and H atoms throughwater ice. From the analysis of the consumption of NO, we estimated the penetra-tion of O or D atoms through porous water ice. We proceed new experiments onthe hydrogenation of NO and found that there is an activation barrier at HNO H step. However, H atoms can cross the activation barrier via quantum tunnelingon the cold surface maintained at 8 K. We indicated that hydroxylamine (NH2OH)is the major product of the hydrogenation of NO at low temperatures, but N2O isthe major product at high temperatures. We also present possible chemical path-ways of the pre-biotic formamide (NH2CHO) through the co-hydrogenation of NOand H2CO on the cold surface (typically at 10 K). Simultaneously, we analysedand discussed the efficient formation route of formamide on interstellar grains. We showed that the chemical network of the hydrogenation of CH3CN and CH3NC iscomplex and takes place via different mechanisms which tend to be competitive.We measured and indicated that the hydrogenation is the dominant mechanism.Furthermore, H2O is always raising the reactive efficiency.
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Hoang Phuong Nguyen Thanh. Experimental study of desorption and reactivity of nitrogen bearing species on interstellar dust grains. Solar and Stellar Astrophysics [astro-ph.SR]. Université de Cergy Pontoise, 2018. English. ⟨NNT : 2018CERG0977⟩. ⟨tel-02286147⟩

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