Exact quantum, quasiclassical, and semiclassical reaction probabilities for the collinear F+H₂ → FH+H reaction

Exact quantum, quasiclassical, and semiclassical reaction probabilities and rate constants for the collinear reaction F+H₂ → FH+H are presented and compared. The exact quantum results indicate a large degree of population inversion of the FH product with P₀₂^R and P₀₃^R being the dominant reaction probabilities. The energy dependence of these two probabilities at low translational energies are quite different. P₀₂^R shows an effective threshold of 0.005 eV which can largely be interpreted as resulting from tunneling through a vibrationally adiabatic barrier. P₀₃^R has a much larger effective threshold (0.045 eV) apparently resulting from dynamical effects. Quasiclassical probabilities for the collinear F+H₂ reaction were calculated by both the forward (initial conditions chosen for reagent F+H ₂) and reverse (initial conditions for product H+FH) trajectory methods. The results of both calculations correctly indicate that P ₀₃^R and P₀₂^R should be the dominant reaction probabilities. However, the threshold behavior of the quasiclassical forward P₀₃^R disagrees strongly with the corresponding exact quantum threshold energy dependence. By contrast, there is good agreement between the reversed trajectory results and the exact quantum ones. The uniform semiclassical results also agree well with the corresponding exact quantum ones indicating that the quasiclassical reverse and the semiclassical methods are preferable to the quasiclassical forward method for this reaction. The important differences between the threshold behavior of the exact quantum and quasiclassical forward reaction probabilities are manifested in the corresponding rate constants primarily as large differences in their activation energies. Additional exact quantum results at higher total energies indicate that threshold effects are no longer important for reactions with vibrationally excited H₂. Resonances play an important role in certain reaction probabilities primarily at higher relative translational energies.