| Description |
In barrier driven reactions, there will be a strong barrier suppression effect for collision energies below the top of the barrier. Thus, in the ultracold temperature limit, one expects the reaction rate for different isotopes to depend in a simple way on their masses. However, there is another important factor that needs to be considered: threshold resonances, i.e. the presence of a virtual or bound state very close to threshold associated with the van der Waals well in the entrance channel. Near a threshold resonance, the scattering length of the reactive collision becomes huge and zero-energy rate coefficient is enhanced by many orders of magnitude. The Wigner threshold behavior becomes restricted to very deep end of the ultracold temperature regime, giving rise to an intermediate regime where the reaction rate coefficient becomes inversely proportional to the temperature. This tremendous increase in the rate coefficient can be rationalized analytically considering the effect of the resonance pole in reactive scattering dynamics. We demonstrate this effect with three benchmark chemical reactions X + H2 (X = D, Cl, F), by theoretically tuning the reduced mass of these systems.
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