Precision test of statistical dynamics with state-to-state ultracold chemistry

TL;DR

This study measures the full product state distribution in ultracold chemical reactions, providing detailed experimental data that tests and challenges existing quantum scattering theories.

Contribution

It offers the first complete measurement of all rotational state-pairs in an ultracold reaction, serving as a benchmark for quantum dynamics calculations.

Findings

Overall agreement with statistical theory

Identification of deviations in certain state-pairs

Precise measurement of the exoergicity limit

Abstract

Chemical reactions represent a class of quantum problems that challenge both the current theoretical understanding and computational capabilities. Reactions that occur at ultralow temperatures provide an ideal testing ground for quantum chemistry and scattering theories, as they can be experimentally studied with unprecedented control, yet display dynamics that are highly complex. Here, we report the full product state distribution for the reaction 2KRb $\rightarrow$ K$_2$ + Rb$_2$. Ultracold preparation of the reactants grants complete control over their initial quantum degrees of freedom, while state-resolved, coincident detection of both products enables the measurement of scattering probabilities into all 57 allowed rotational state-pairs. Our results show an overall agreement with a state-counting model based on statistical theory, but also reveal several deviating state-pairs. In particular, we observe a strong suppression of population in the state-pair closest to the exoergicity limit, which we precisely determine to be $9.7711^{+0.0007}_{-0.0005}$ cm$^{-1}$, as a result of the long-range potential inhibiting the escape of products. The completeness of our measurements provides a valuable benchmark for quantum dynamics calculations beyond the current state-of-the-art.