Threshold resonance effects in reactive processes

We investigate the effect of near threshold resonances in reactive scattering at low energy. We find a general type of anomalous behavior of the cross sections, and illustrate it with a real system (H2+Cl). For inelastic processes, the anomalous energy dependence of the total cross section is given by σ∼k-3 (with k). The standard threshold behavior given by Wigner's law (σ∼k-1) is eventually recovered at vanishing energies, but its validity is now limited to a much narrower range of energies. The anomalous behavior leads to reaction rate coefficients behaving as K∼1/T instead of the expected constant rate of the Wigner regime. We also provide analytical expressions for s-wave cross sections, and discuss the implication in ultracold physics and chemistry.