Theoretical study of collisions in the interstellar medium

The Herschel Space Observatory satellite has permitted to detect light in the far infrared, corresponding to frequencies at which molecules emit light through rotational transitions. ALMA, an interferometer located on the Chilean desert of Atacama took over Herschel and will allow new observations of cold molecular clouds with an accurate angular accuracy. ALMA observes in the range of millimeter and sub-millimeter which makes it complementary to the Satellite Herschel (far infrared). These significant advances in observing particle interactions at the microscopic level, to produce and trap diatomic molecules in specific internal states, open new perspectives in the field of collision physics and theoretical chemistry. Observation of interstellar molecules has benefited from advances in astronomy, to identify vibrational rotational transitions of molecules. Furthermore, spectroscopic data provide us with important information on the state of the interstellar medium: ionized or neutral. The cation methylidene CH⁺ was observed in the diffuse medium through its X1Σ + -A1π electronic transition. It plays an important role in the different stages of the complex chemical behaviour through processes and molecular reactions that occur in interstellar and circumstellar regions. So CH⁺ launches large chain chemical processes that can progress to the formation of more complex species. The fine structure transition of C⁺ is the strongest emission line of the Milky Way. The C⁺ ion is a tracer of density and temperature in diffuse clouds and regions dominated by photons (PDR). The C⁺ line is an important tool to probe the gas content and star formation processes in the Milky Way and other galaxies. C⁺ + H₂ collisions can lead to the formation of CH⁺. This reaction has been extensively studied theoretically and experimentally, however, it is endothermic by 3211cm^-1 and at the typical temperatures for MIS and H₂ in its ground vibrational state, the reaction does not occur. The only process is then the C⁺ spin-orbit excitation process. Spin orbit relaxation C + (2P1/2) + H 2 (v j) = C + (2P3/2) + H₂ (v0; j0) which was first studied in this thesis contributes to the cooling of the gas constituting the interstellar clouds. The vibrational excitation of H₂ (v≥ 0), which takes place during collisions with C⁺ has a significant influence on the abundance of CH⁺. CH⁺ is a highly reactive ion, it is destroyed by the abstraction reaction of hydrogen that has been considered in this work. It is therefore interesting to accurately assess the effectiveness of this path of destruction. The dilemma is that this ion is also abundantly found in the neutral and cold environment. This thesis focuses on the inelastic and reactive collisions studies of interstellar interest. We used ab initio highly correlated methods to tackle the electronic structure parts. Moreover, the nuclear dynamics of the systems was studied using a time independent quantum formalism, based on the Jacobi coordinates in the case of the spin-orbit excitation of C⁺ (2P) + ortho H₂, and para-H₂ and rotational excitation of (+ CD) + He, or the hyper spherical coordinates for the reactive process in the case of the abstraction of a hydrogen in H⁺ CH⁺. Our concern was to give a comprehensive basis of the mechanisms and provide a quantification of the effective spin-orbit relaxation cross sections and reaction rates to confront with spectroscopic observations. The new rate constants we obtained should help to better interpret the observations of C⁺ radiation emissions obtained by current and future telescopes. (author)