The role of s-wave scattering in atom-ion resonant charge transfer near room temperature*

Resonant exchange is a general process playing a key role in many-body dynamics and transport phenomena, such as spin, charge, or excitation diffusion. A particular example is given by the collision between an ion and its parent neutral atom. The underlying process is described by the resonant exchange cross section. We show that the s-wave scattering, generally thought to contribute mainly in the ultracold (or Wigner) regime, dictates the overall cross section and corresponding rate coefficient over a broad range of energies. We derive an analytical expression and explain its applicability high above the Wigner regime, and demonstrate its relationship to the classical capture (Langevin) cross section. We apply it to resonant charge transfer for several isotopes of Yb, and show that fitting higher energy data allows to obtain the cross section and rate over a large range of energies down to the ultracold regime.