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 (H-2 + Cl). For inelastic processes, the anomalous energy dependence of the total cross section is given by sigma similar to k(-3) (with k proportional to root epsilon). The standard threshold behavior given by Wigner's law (sigma similar to 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 similar to 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.