Spin-orbit transitions in the N$^+$($^3P_{J_A}$) + H$_2$ $\rightarrow$ NH$^+$($X^2\Pi$, $^4\Sigma^-$)+ H($^2S$) reaction, using adiabatic and mixed quantum-adiabatic statistical approaches

Abstract

The cross section and rate constants for the title reaction are calculated for all the spin-orbit states of N$^+$($^3P_{J_A}$) using two statistical approaches, one purely adiabatic and the other one mixing quantum capture for the entrance channel and adiabatic treatment for the products channel. This is made by using a symmetry adapted basis set combining electronic (spin and orbital) and nuclear angular momenta in the reactants channel. To this aim, accurate {\it ab initio} calculations are performed separately for reactants and products. In the reactants channel, the three lowest electronic states (without spin-orbit couplings) have been diabatized, and the spin-orbit couplings have been introduced through a model localizing the spin-orbit interactions in the N$^+$ atom, which yields accurate results as compared to {\it ab initio} calculations including spin-orbit couplings. For the products, eleven purely adiabatic spin-orbit states have been determined with {\it ab initio} calculations. The reactive rate constants thus obtained are in very good agreement with the available experimental data for several ortho-H$_2$ fractions, assuming a thermal initial distribution of spin-orbit states. The rate constants for selected spin-orbit $J_A$ states are obtained, to provide a proper validation of the spin-orbit effects to obtain the experimental rate constants.