Charge transfer in sodium iodide collisions.

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.

Mutual neutralization in collisions of H+ with Cl.

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.

Reactions of C+ + Cl-, Br-, and I--A comparison of theory and experiment.

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.