  Chemical reactions at ultracold temperature provide an ideal platform to
study chemical reactivity at the fundamental level, and to understand how
chemical reactions are governed by quantum mechanics. Recent years have
witnessed the remarkable progress in studying ultracold chemistry with
ultracold molecules. However, these works were limited to exothermic reactions.
The direct observation of state-to-state ultracold endothermic reaction remains
elusive. Here we report on the investigation of endothermic and nearly
thermoneutral atom-exchange reactions in an ultracold atom-dimer mixture. By
developing an indirect reactant-preparation method based on a molecular
bound-bound transition, we are able to directly observe a universal endothermic
reaction with tunable energy threshold and study the state-to-state reaction
dynamics. The reaction rate coefficients show a strikingly threshold
phenomenon. The influence of the reverse reaction on the reaction dynamics is
observed for the endothermic and nearly thermoneutral reactions. We carry out
zero-range quantum mechanical scattering calculations to obtain the reaction
rate coefficients, and the three-body parameter is determined by comparison
with the experiments. The observed endothermic and nearly thermoneutral
reaction may be employed to implement collisional Sisyphus cooling of
molecules, study the chemical reactions in degenerate quantum gases and conduct
quantum simulation of Kondo effect with ultracold atoms.
