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.
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