Non-adiabatic interactions in small molecular systems

Collisions of charged particles i.e. atomic ions, molecular ions and electrons, are important to understand in cold plasma environments such as the interstellar medium. A fully quantum mechanical description of such reactions are possible for small molecular systems, where the Born-Oppenheimer treatment allows for the molecular problem to be solved in two separate steps. In a rst step the electronic motion is solved for giving rise to the concepts of adiabatic potential energy surfaces and non-adiabatic coupling, which in turn determine the motion of the nuclei for the molecular system. This thesis includes two projects. In a rst project mutual neutralization in C++Cl  and H+ + Cl  collisions are studied. Thermal rate coecient for the reactions have been measured which allows for a comparison between theory and experiment. Theoretically, these reactions are driven by the non- adiabatic couplings at a small internuclear distance. The cross section and thermal rate for the reactions are calculated and the e
ect of spin-orbit coupling is estimated using a semi-empirical method. In the second project the electronic resonant states in H3 are studied. A generalized complex pseudo Jahn-Teller model is introduced to describe the interaction among these resonant states. The model parameters are extracted by tting the model to electron scattering calculations for the H3+e system. Non-adiabatic coupling and geometric phase is further calculated in order to characterise the adiabatic potential energy surfaces of the system.