Cold and ultracold NH--NH collisions: The field-free case

TL;DR

This study calculates elastic and inelastic collision cross sections for cold NH molecules, showing potential for successful evaporative cooling in magnetic trapping experiments, despite sensitivity to interaction potential details.

Contribution

First full quantum scattering calculations on an accurate ab initio potential for NH--NH collisions at ultracold temperatures, assessing cooling feasibility.

Findings

Elastic-to-inelastic cross-section ratio favorable for cooling

Cross sections highly sensitive to potential energy surface details

Evaporative cooling likely feasible for certain hyperfine states

Abstract

We present elastic and inelastic spin-changing cross sections for cold and ultracold NH($X\,^3\Sigma^-$) + NH($X\,^3\Sigma^-$) collisions, obtained from full quantum scattering calculations on an accurate \textit{ab initio} quintet potential-energy surface. Although we consider only collisions in zero field, we focus on the cross sections relevant for magnetic trapping experiments. It is shown that evaporative cooling of both fermionic $^{14}$NH and bosonic $^{15}$NH is likely to be successful for hyperfine states that allow for s-wave collisions. The calculated cross sections are very sensitive to the details of the interaction potential, due to the presence of (quasi-)bound state resonances. The remaining inaccuracy of the \textit{ab initio} potential-energy surface therefore gives rise to an uncertainty in the numerical cross-section values. However, based on a sampling of the uncertainty range of the \textit{ab initio} calculations, we conclude that the exact potential is likely to be such that the elastic-to-inelastic cross-section ratio is sufficiently large to achieve efficient evaporative cooling. This likelihood is only weakly dependent on the size of the channel basis set used in the scattering calculations.