State-selected chemical reaction dynamics at the S matrix level: Final-state specificities of near-threshold processes at low and high energies

State-to-state reaction probabilities are found to be highly final-state specific at state-selected threshold energies for the reactions O + H ₂ → OH + H and H + H₂ → H₂ + H. The study includes initial rotational states with quantum numbers 0-15, and the specificity is especially dramatic for the more highly rotationally excited reactants. The analysis is based on accurate quantum mechanical reactive scattering calculations. Final-state specificity is shown in general to increase with the rotational quantum number of the reactant diatom, and the trends are confirmed for both zero and nonzero values of the total angular momentum.