\"Uberresonant Scattering of Ultracold Molecules

TL;DR

Ultracold molecular collisions exhibit a dense spectrum of Fano-Feshbach resonances, which are so numerous and broad that they effectively average out, simplifying the scattering behavior to non-resonant models.

Contribution

This work demonstrates that the high density and broadening of resonances in ultracold molecular collisions make individual resonances unresolvable, leading to effective non-resonant scattering descriptions.

Findings

Resonance density in ultracold molecules exceeds experimental resolution.

Resonance broadening due to chemical reactions renders individual resonances unobservable.

Simplified non-resonant scattering models are sufficient for describing ultracold molecular collisions.

Abstract

Compared to purely atomic collisions, ultracold molecular collisions potentially support a much larger number of Fano-Feshbach resonances due to the enormous number of ro-vibrational states available. In fact, for alkali-metal dimers we find that the resulting density of resonances cannot be resolved at all, even on the sub-$\mu$K temperature scale of ultracold experiments. As a result, all observables become averaged over many resonances and can effectively be described by simpler, non-resonant scattering calculations. Two particular examples are discussed: non-chemically reactive RbCs and chemically reactive KRb. In the former case, the formation of a long-lived collision complex may lead to the ejection of molecules from a trap. In the latter case, chemical reactions broaden the resonances so much that they become unobservable.