Cette thèse a été effectuée au sein de l’Institut de Physique de Rennes, et qui porte sur le transfert d’énergie rotationnelle lors des collisions de CO-Ar et CO-H2 à très basses températures pour des applications astrophysiques. Comprendre la constitution du milieu interstellaire (MIS), son évolution et ses propriétés physiques telles que la température et la densité, nécessite la connaissance de l’efficacité des collisions atomiques et moléculaires. Cette thèse expérimentale a été motivée par cet objectif. Le MIS et plus particulièrement les nuages moléculaires froids sont caractérisés par des températures très basses atteignant ~ -260 °C. Afin de reproduire ces conditions, on a utilisé la technique CRESU (Cinétique des Réactions en Ecoulement Supersonique Uniforme). Deux systèmes de collisions ont été étudiés : CO-Ar et CO-H2 pour leur impact dans les modèles astrophysiques (dits aussi modèles de transfert radiatifs). Une technique spectroscopique IR-VUV (Infrarouge-Ultraviolet dans le vide) en double résonance à base de lasers pulsés a été utilisée pour la détection et le diagnostic de l’efficacité des collisions et la détermination les constantes de collisions. Les résultats expérimentaux obtenus (pour la première fois) ont été comparés à des prédictions basées sur des calculs théoriques très avancés de mécanique quantique. Un très bon accord a été obtenu, ce qui a permis de tester et valider ces calculs théoriques d’un côté, et aussi de pouvoir fournir des constantes de collisions robustes qui vont être utilisées par les astrophysiciens pour modéliser et déterminer les propriétés physiques du MIS, ainsi qu’interpréter les spectres astrophysiques obtenus par des télescopes ou des satellites. / In the quest to understand the universe, astrophysicists observe and make models for astrophysical objects in the sky. The interstellar medium, ISM, in particular is of central importance since it represents the matter that exists in the space between stars in a galaxy, and in which stars and planets form. Understanding it, its constituents and its evolution and characteristics requires the quantification of several chemical and physical processes, including collision processes. In this work, we used the CRESU technique to reproduce very cold environments of astrophysical media, in particular dense molecular clouds in the ISM. We studied experimentally rotational energy transfer, RET, resulting from inelastic collisions at very low temperatures using a pump-probe laser-based spectroscopic technique for the purpose of measuring constants quantifying collisions. Two types of constants were determined: the first are total removal constants of RET resulting from a specific rotational quantum state to all possible final rotational quantum states, and the second are more detailed information consisting in rate constants from a specific rotational quantum state to another specific rotational quantum state, so-called state-to-state rate constants. Two experiments have been performed involving Carbone monoxide molecule, CO, as a target molecule of collisions. The first involves argon, Ar, as a projectile atom, and the second molecular hydrogen, H2, as a projectile molecule. Both collisional systems play an important role in a wide range of areas including gas-phase phenomena and astrophysical applications. In the first experiment, we investigated collisions between CO and Ar, from 293 K down to 30 K. IR-VUV double resonance technique has been exploited to measure, directly for the first time, absolute values of total removal and state-to-state constants of collisions. The experimental results have been compared to theoretical predictions based on a diatom-atom model of collision, where very good agreement was observed. In the second experiment, we investigated collisions between CO and H2 (the most abundant molecules in the ISM) from 293 K down to 5.5 K focusing on the very low temperatures of dense molecular clouds in the ISM. For the first time, total removal and state-to-state constants have been measured and compared to theoretical predictions of a highly accurate diatom-diatom model of collisions, where excellent agreement was observed. The results obtained in this thesis served to validate theoretical models of molecular collisions, helping the continuous efforts for pushing the frontiers of theoretical models. In the astrophysical side, the obtained collisional constants will be taken into account in modeling of many astrophysical media.
| Identifer | oai:union.ndltd.org:theses.fr/2017REN1S147 |
| Date | 19 December 2017 |
| Creators | Labiad, Hamza |
| Contributors | Rennes 1, Sims, Ian |
| Source Sets | Dépôt national des thèses électroniques françaises |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation, Text |
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