Ultracold collisions in magnetic fields: reducing inelastic cross sections near Feshbach resonances

TL;DR

This study investigates how magnetic fields influence ultracold He + NH collisions, revealing that tuning near Feshbach resonances can reduce inelastic losses, aiding molecular cooling techniques.

Contribution

It provides detailed quantum scattering calculations identifying Feshbach resonances and their effects on inelastic cross sections in ultracold molecular collisions.

Findings

Feshbach resonances cause asymmetric oscillations in scattering length.

Inelastic cross sections show troughs near resonances, indicating reduced losses.

Scattering length does not pass through a pole at resonance.

Abstract

We have carried out bound-state and low-energy quantum scattering calculations on He + NH (triplet Sigma) in magnetic fields, with the NH molecule in its n=1 rotationally excited states. We have explored the pattern of levels as a function of magnetic field and identified the nearly good quantum numbers in different regimes. We have used the bound-state calculations to locate low-energy Feshbach resonances. When the magnetic field is used to tune across such a resonance, the real and imaginary part of the scattering length show asymmetric oscillations and peaks with amplitude between 1 and 3 Angstroms. The scattering length does not pass through a pole at resonance. The resonant behavior is characterized by a complex resonant scattering length a_res. The corresponding inelastic cross sections show troughs as well as peaks near resonance. This may be important for efforts to achieve evaporative and sympathetic cooling for molecules, because it offers the hope that inelastic trap losses can be reduced by tuning close to a Feshbach resonance.