Advanced computational studies of collisions of complex atoms, ions, and molecules
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This dissertation focuses on the theoretical investigation of non-adiabatic collision processes of complex atoms, ions, and molecules. The inelastic processes, particularly charge transfer due to ion-atom collisions and quenching and excitation in atom-atom collisions, are studied using a quantum-mechanical molecular-orbital close-coupling method, which is based on the perturbed stationary state approach adopting molecular orbitals as basis functions. Within this collision model, the motion of nuclei is governed by adiabatic potential energy surfaces which are constructed from the motion of electrons. Transitions between adiabatic molecular states are driven by non-adiabatic couplings. With the adiabatic potential energies and non-adiabatic couplings provided by the multireference single- and double excitation con figuration interaction method, a set of coupled Schrodinger equations is solved to obtain the collisional cross sections. We discuss the theoretical method in detail giving the coupled-channel equations in the adiabatic and diabatic representations. A transformation between the adiabatic and diabatic pictures are described. The partial wave analysis to obtain radial coupled equations and the resulting S-matrix is discussed. In order to extend the current theoretical method for ion-atom collisions to ion-molecule collisions, the in finite order sudden approximation is adopted to reduce the complexity arising from the rotational motion of molecular targets. Applications of these methods to three di fferent collision systems are given. In N-H+ collisions, rate coefficients, total and state-selective cross sections for electron capture processes are presented. For Na-He collisions, collisional cross sections and rate coefficients for elastic scattering and inelastic quenching and excitation are given. Additionally, the variation of scattering lengths with reduced mass and collision energy and their relation to vibrational bound states of the quasi-molecule are illustrated. Finally, for H+-CO collisions, we calculate vibrationally-resolved cross sections elucidating vibronic transitions for three di fferent orientation angles. Angle-averaged results are given. The steric eff ect is prominent in the angle-dependent results.