The problem of asymptotic non-adiabatic couplings in heavy particle collisions is treated using the reprojection method. The mixing matrix that mixes the asymptotic solutions of the coupled states to obtain appropriate boundary conditions is here derived to second order, yielding a faster convergence of the cross section. In addition, the reprojection method is implemented in a diabatic representation and applied to inelastic scattering of Li + Na and H + H collisions and to mutual neutralization in H⁺ + H⁻ collisions.

Associative ionization in collisions of H+ + H- as well as H(1s) + H(ns) with n = 2, 3, 4 is studied theoretically. Relevant adiabatic potential curves and nonadiabatic couplings are calculated ab initio and the autoionization from the lowest electronic resonant states in the 11+g/u and 31+g/u symmetries are considered. The cross sections are obtained by solving the coupled Schrodinger equation, including a complex potential matrix, in a strict diabatic representation. The importance of using a nonlocal description of autoionization is investigated. Associative ionization is also studied for different isotopes of hydrogen. Calculated cross sections are compared with results from measurements.

Sodium iodide (NaI) has, over the years, served as a prototype system in studies of non-adiabatic dynamics. Here, the charge transfer collision reactions Na⁺ + I− ⇆ Na + I (mutual neutralization and ion-pair formation) are studied using an ab initio approach and the total and differential cross sections are calculated for the reactions. This involves electronic structure calculations on NaI to obtain adiabatic potential energy curves, non-adiabatic and spin–orbit couplings, followed by nuclear dynamics, treated fully quantum mechanically in a strictly diabatic representation. A single avoided crossing at 13.22 a₀ dominates the reactions, and the total cross sections are well captured by the semi-classical Landau–Zener model. Compared to the measured ion-pair formation cross section, the calculated cross section is about a factor of two smaller, and the overall shape of the calculated differential cross section is in reasonable agreement with the measured ion-pair formation differential cross section. Treating the Landau–Zener coupling as an empirical parameter of 0.05 eV, the measured total and differential cross sections are well captured when performing fully quantum mechanical cross section calculations including rotational coupling. A semi-empirical spin–orbit coupling model is also investigated, giving satisfactory estimation of the effects of spin–orbit interactions for the reactions. 

In this thesis, theoretical ab initio treatments of two-body molecular collision reactions are studied, having in common that the interaction region including all coupling mechanisms driving the reaction amounts to a molecular description. The main goal is to gain an understanding in the underlying coupling mechanisms involved in these reactions.The thesis is divided into three projects. In project one, mutual neutralization in collisions of Na⁺ + I⁻, C⁺ + Cl⁻ and H⁺+H⁻ are studied, with an emphasis on the inclusion of spin-orbit and/or rotational couplings which are most often neglected for in mutual neutralization. Scattering quantities are computed ab initio and compared to approximative models and experimental results. In project two, the problem of asymptotic non-adiabatic couplings is studied. Specifically, the inclusion of higher order terms in the reprojection method is shown to give a much faster convergence of the relevant scattering cross section. The method is here applied to mutual neutralization in H⁺+H⁻collisions and inelastic scattering in Li+Na and H+H collisions. In project three, a generalized pseudo Jahn-Teller model is introduced an applied to electronic resonant states of H₃. Model parameters are extracted using electron scattering calculations resultingin a non-Hermitian Hamiltonian describing the system. The topology of the resulting complex adiabatic potential energy surfaces, including complex conical intersections and non-Hermitian degeneracies, are furthermore studied and classified.

The total and differential cross sections of mutual neutralization in H⁺+H⁻ collisions are calculated ab initio and fully quantum mechanically for energies between 0.001 and 600 eV. Effects which have not previously been considered in studies on mutual neutralization (MN) for this system, such as inclusion of rotational couplings and autoionization, are investigated. Adiabatic potential curves corresponding to the relevant states of ¹Σ, ¹Σ, ¹Π_g and ¹Π_u symmetries as well as radial and rotational nonadiabatic couplings are computed ab initio. A quasidiabatic model is developed and applied in order to investigate the importance of higher excited states as well as the inclusion of autoionization. Molecular data for the lowest electronic resonant state in each symmetry are obtained by performing electron scattering calculations. It is shown that rotational couplings cause a significant increase of the total MN cross section while autoionization plays a minor role as a loss mechanism. Additionally, a differential cross section is obtained that is symmetric around θ=90^∘. This result is in disagreement with a previous theoretical calculation where it was found that the differential cross section is dominated by backwards scattering.

We study the electronic resonant states of H₃ with energies above the potential energy surface of the H_3⁺ ground state. These resonant states are important for the dissociative recombination of H_3⁺ at higher collision energies, and previous studies have indicated that these resonant states exhibit a triple intersection. We introduce a complex generalization of the pseudo–Jahn-Teller model to describe these resonant states. The potential energies and the autoionization widths of the resonant states are computed with electron scattering calculations using the complex Kohn variational method, and the complex model parameters are extracted by a least-square fit to the results. This treatment results in a non-Hermitian pseudo–Jahn-Teller Hamiltonian describing the system. The nonadiabatic coupling and geometric phase are further calculated and used to characterize the enriched topology of the complex adiabatic potential energy surfaces.

The cross section and final state distribution for mutual neutralization in collisions of H+ with Cl- have been calculated using an ab initio quantum mechanical approach. It is based on potential energy curves and nonadiabatic coupling elements for the six lowest (1)Sigma(+) states of HCl computed with the multireference configuration interaction method. The reaction is found to be driven by nonadiabatic interactions occurring at relatively small internuclear distances (R < 6 a(0)). Effects on the mutual neutralization cross section with respect to the asymptotic form of the potential energy curves, inclusion of closed channels, as well as isotopic substitution are investigated. The effect of spin-orbit interaction is investigated using a semiempirical model and found to be small. A simple two-state Landau-Zener calculation fails to predict the cross section.

Rate constants for the reactions of C+ + Cl-, Br-, and I- were measured at 300 K using the variable electron and neutral density electron attachment mass spectrometry technique in a flowing afterglow Langmuir probe apparatus. Upper bounds of <10(-8) cm(3) s(-1) were found for the reaction of C+ with Br- and I-, and a rate constant of 4.2 +/- 1.1 x 10(-9) cm(3) s(-1) was measured for the reaction with Cl-. The C+ + Cl- mutual neutralization reaction was studied theoretically from first principles, and a rate constant of 3.9 x 10(-10) cm(3) s(-1), an order of magnitude smaller than experiment, was obtained with spin-orbit interactions included using a semiempirical model. The discrepancy between the measured and calculated rate constants could be explained by the fact that in the experiment, the total loss of C+ ions was measured, while the theoretical treatment did not include the associative ionization channel. The charge transfer was found to take place at small internuclear distances, and the spin-orbit interaction was found to have a minor effect on the rate constant.

Associative ionization in collisions of H+ + H− as well as H(1s) + H(ns) with n = 2, 3, 4 is studiedtheoretically. Relevant adiabatic potential curves and non-adiabatic couplings are calculated abinitio and the autoionization from the lowest electronic resonant states in the 1Σ+g/u and 3Σ+g/usymmetries are considered. The cross sections are obtained by solving the coupled Schr¨odingerequation, including a complex potential matrix, in a strict diabatic representation. The importanceof using a non-local description of autoionization is investigated. Associative ionization is alsostudied for different isotopes of hydrogen. Calculated cross sections are compared with results frommeasurements.

The problem of asymptotic non-adiabatic coupling is treated using the reprojection method.In contrast to previous studies, the mixing matrix is derived to second order in 1/R yielding afaster convergence of the cross section. The reprojection method is here implemented in a diabaticrepresentation and applied to inelastic scattering of Li+Na, inelastic scattering of H+H and mutualneutralization in H++H− collisions.