Abstract :

The chemical composition of the interstellar medium (ISM) is increasingly well known, thanks to observations that have led to the discovery of a growing number of new molecular species. In order to understand the evolution of the various environments of the ISM, it is necessary to use models consisting of chemical networks describing the formation and destruction pathways of the species observed. To build such models, it is necessary to study processes in order to provide quantitative information describing chemical reactions. The chemistry of the ISM relies heavily on ion-molecule reactions, which can be studied using mass spectrometers, that occur without activation barrier and therefore easily take place in the cold environment of the ISM.

During my thesis work, the CERISES instrument was used to study cation-molecule reactions. This instrument, which is a guided ion beam mass spectrometer, allows control of the energy imparted to the reactants at the time of the reaction. The internal energy of the ions can be controlled, using VUV photons from the SOLEIL synchrotron as the ionization source, and the collision energy, via the kinetic energy imparted to the parent ions before the collision.

The reactivity of the two isomers of protonated carbon monoxide, HCO⁺ and HOC⁺, was studied using 14 different targets. These targets, all present in the ISM, were used to establish a scale of proton affinity and ionization energy. Two different precursors were used to generate HCO⁺ and HOC⁺ ions by dissociative photoionisation: formaldehyde, H₂CO, which generates a pure population of HCO⁺ ions, and deuterated methanol, CD₃OH, which generates a population of over 90 % pure HOC⁺ ions. These measurements, carried out on the DESIRS line at the SOLEIL synchrotron, highlighted proton transfer as the main reaction for both ions. The experimental thresholds observed as a function of photon energy for this reaction are consistent with the thresholds expected by combining the enthalpies of reaction and the energy of appearance of the two cations. The study of the reactivity of DCO⁺, the deuterated form of HCO⁺, enabled us to determine the protonation sites of the targets studied and to assign the products of dissociative proton transfer. The study of collision-induced isomerization of HOC⁺ to HCO⁺ was carried out using an innovative method combining both time of flight measurements of the ions inside the CERISES instrument and simulations of the catalyzed isomerization process using the SIMION software.

The reactivity of the CN^- and C₃N^- anions was also explored with four oxygenated targets: formic and acetic acids, acetaldehyde and methanol. The two parent anions were generated using dissociative electronic attachment of BrCN and BrC₃N, respectively. The development of a time-stable and intense anion generation method within the CERISES instrument required many instrumental developments. A proton transfer reaction is observed for the two parent anions with the four targets. This reaction exhibits reactivity thresholds as a function of collision energy that are consistent with the enthalpies of reaction. An oxygen transfer reaction was also observed. This redox reaction of CN^- and C₃N^- anions leads to the formation of OCN^- or OC₃N^-, respectively. OCN^- formation is only observed for CN^- with methanol and acetaldehyde, which exhibit endothermic proton transfer. The formation of OC₃N^- from C₃N^- was observed with all four targets studied. This revealed competition between the proton transfer and oxygen transfer reactions. It also indicates the formation of a different ion-molecule complex between the two parent anions.