Robin Côté "Signature of the s-wave scattering due to locking and unlocking of phase shifts [...]"

Invited talk by Robin Côté (University of Massachusetts Boston, USA):
"Signature of the s-wave scattering due to locking and unlocking of phase shifts in resonant-exchange processes"
This talk was a part of Workshop on Cold Hybrid Ion-Atom Systems 2022, that took place in Warsaw, Poland on 08-10.06.2022.
For more details please refer to workshop website: ion-atom-2022....
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Abstract:
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 of such a process is the resonant charge transfer between an ion and its parent atom. The underlying dynamics 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 over a broad range of energies. In fact, this counter-intuitive behavior can be understood as an unexpected consequence of phase-shift locking; namely, all phase-shifts are “locked” relative to each other over a large range of energies, leading to a quasi-constant phase-shift difference and hence a simple expression for the resonant exchange cross section. We also explore the regime when this is followed by a phase-shift “unlocking” taking place at higher energies. We derive an analytical expression and explain its applicability over a large range of collision energy, high above the Wigner regime; it accounts not only for the locking of phase shifts, but also for their gradual unlocking as the energy increases. We demonstrate the relationship to the classical capture (Langevin) cross section and apply it to three very different resonant processes: namely, resonant charge transfer, spin flip, and excitation exchange. Finally, we apply the expression to the case of resonant charge-transfer in ion-atom collisions over a large range of energies up to room temperatures and find good agreement between the computed (fully quantal) cross section and our newly obtained result. The s-wave signature also allows us to gain information about the Wigner regime from data obtained at much higher temperatures, which is especially advantageous for systems where the ultracold regime is not reachable. We show how one could use information at higher energies to gain insight into the s-wave regime.