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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The mutual neutralization reaction in collisions of Li+ with CN− is a promising candidate for rigorous multi-dimensional ab initio studies of atom-molecule charge transfer processes. The reaction is driven by the non-adiabatic interaction between the lowest two 1A′ electronic states at large Li–CN distances, resulting in a large cross section for mutual neutralization. As a first step, the relevant adiabatic potential energy surfaces and non-adiabatic interaction are computed ab initio, and the process is studied quantum mechanically using the vibrational sudden approximation, where the vibrational and rotational motions of the CN molecule are assumed to be frozen during the collision. [ABSTRACT FROM AUTHOR]

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 cm3 s−1 were found for the reaction of C+ with Br− and I−, and a rate constant of 4.2 ± 1.1 × 10−9 cm3 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 × 10−10 cm3 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. [ABSTRACT FROM AUTHOR]

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 1Σ+ 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 a0). 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. [ABSTRACT FROM AUTHOR]

The dissociative recombination of HCl+, including both the direct and indirect mechanisms, is studied. For the direct process, the relevant electronic states are calculated ab initio by combining electron scattering calculations to obtain resonance positions and autoionization widths with multi-reference configuration interaction calculations of the ion and Rydberg states. The cross section for the direct dissociation along electronic resonant states is computed by solution of the time-dependent Schrüdinger equation. For the indirect process, an upper bound value for the cross section is obtained using a vibrational frame transformation of the elements of the scattering matrix at energies just above the ionization threshold. Vibrational excitations of the ionic core from the ground vibrational state, v = 0, to the first three excited vibrational states, v = 1, v = 2, and v = 3, are considered. Autoionization is neglected and the effect of the spin-orbit splitting of the ionic potential energy upon the indirect dissociative recombination cross section is considered. The calculated cross sections are compared to measurements. [ABSTRACT FROM AUTHOR]

The dissociative recombination of HCl+, including both the direct and indirect mechanisms, is studied. For the direct process, the relevant electronic states are calculated ab initio by combining electron scattering calculations to obtain resonance positions and autoionization widths with multi-reference configuration interaction calculations of the ion and Rydberg states. The cross section for the direct dissociation along electronic resonant states is computed by solution of the time-dependent Schrüdinger equation. For the indirect process, an upper bound value for the cross section is obtained using a vibrational frame transformation of the elements of the scattering matrix at energies just above the ionization threshold. Vibrational excitations of the ionic core from the ground vibrational state, v = 0, to the first three excited vibrational states, v = 1, v = 2, and v = 3, are considered. Autoionization is neglected and the effect of the spin-orbit splitting of the ionic potential energy upon the indirect dissociative recombination cross section is considered. The calculated cross sections are compared to measurements. [ABSTRACT FROM AUTHOR]

The resonant states of HeH are computed by combining structure calculations at a full configuration interaction level with electron scattering calculations carried out using the Complex-Kohn variational method. We obtain the potential energy curves, autoionization widths, as well as non-adiabatic couplings among the resonant states. Using the non-adiabatic couplings, the adiabatic to diabatic transformation matrix can be obtained. A strict diabatization of the resonant states will be used to study various scattering processes where the resonant states are involved. These processes involve high energy dissociative recombination (DR) and ion-pair formation (RIP), resonant and direct dissociative excitation (DE), penning ionization (PI) as well as mutual neutralization (MN).

Using the Multi-Reference Configuration Interaction method, the adiabatic potential energy surfaces of Li3 are computed. The two lowest electronic states are bound and exhibit a conical intersection. By fitting the calculated potential energy surfaces to the cubic E ⊗ ε Jahn-Teller model we extract the effective Jahn-Teller parameters corresponding to Li3. These are used to set up the transformationmatrix which transforms from the adiabatic to a diabatic representation. This diabatization method gives a Hamiltonian for Li3 which is free from singular non-adiabatic couplings and should be accurate for large internuclear distances, and it thereby allows for bound dynamics in the vicinity of the conical intersection to be explored. [ABSTRACT FROM AUTHOR]

Quantum scattering calculations of two and three-body systems with Coulomb interaction using the driven Schrödinger equation combined with exterior complex scaling are discussed. A rigorous formulation for two-body scattering is reported, and its generalization to three-body scattering is considered. [ABSTRACT FROM AUTHOR]

Using the Multi-Reference Configuration Interaction method, the adiabatic potential energy surfaces of Li3 are computed. The two lowest electronic states are bound and exhibit a conical intersection. By fitting the calculated potential energy surfaces to the cubic E ⊗ ε Jahn-Teller model we extract the effective Jahn-Teller parameters corresponding to Li3. These are used to set up the transformationmatrix which transforms from the adiabatic to a diabatic representation. This diabatization method gives a Hamiltonian for Li3 which is free from singular non-adiabatic couplings and should be accurate for large internuclear distances, and it thereby allows for bound dynamics in the vicinity of the conical intersection to be explored. [ABSTRACT FROM AUTHOR]

The mutual neutralization reaction in collisions of Li + with CN - is a promising candidate for rigorous multi-dimensional ab initio studies of atom-molecule charge transfer processes. The reaction is driven by the non-adiabatic interaction between the lowest two 1 A' electronic states at large Li-CN distances, resulting in a large cross section for mutual neutralization. As a first step, the relevant adiabatic potential energy surfaces and non-adiabatic interaction are computed ab initio , and the process is studied quantum mechanically using the vibrational sudden approximation, where the vibrational and rotational motions of the CN molecule are assumed to be frozen during the collision.

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 × 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 × 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.

The dissociative recombination of HCl + , including both the direct and indirect mechanisms, is studied. For the direct process, the relevant electronic states are calculated ab initio by combining electron scattering calculations to obtain resonance positions and autoionization widths with multi-reference configuration interaction calculations of the ion and Rydberg states. The cross section for the direct dissociation along electronic resonant states is computed by solution of the time-dependent Schrödinger equation. For the indirect process, an upper bound value for the cross section is obtained using a vibrational frame transformation of the elements of the scattering matrix at energies just above the ionization threshold. Vibrational excitations of the ionic core from the ground vibrational state, v = 0, to the first three excited vibrational states, v = 1, v = 2, and  v = 3, are considered. Autoionization is neglected and the effect of the spin-orbit splitting of the ionic potential energy upon the indirect dissociative recombination cross section is considered. The calculated cross sections are compared to measurements.