In many chemical reactions, the transformation from reactants to products is mediated by transient intermediate complexes. For gas-phase reactions involving molecules with a few atoms, these complexes typically live on the order of 10 ps or less before dissociating, and are therefore rarely influenced by external processes. Here, we demonstrate that the transient intermediate complex K₂Rb₂*, formed from collisions between ultracold KRb molecules, undergoes rapid photo-excitation in the presence of a continuous-wave laser source at 1,064 nm, a wavelength commonly used to confine ultracold molecules. These excitations are facilitated by the exceptionally long lifetime of the complex under ultracold conditions. Indeed, by monitoring the change in the complex population after the sudden removal of the excitation light, we directly measure the lifetime of the complex to be 360 ± 30 ns, in agreement with our calculations based on the Rice–Ramsperger–Kassel–Marcus (RRKM) statistical theory. Our results shed light on the origin of the two-body loss widely observed in ultracold molecule experiments. Additionally, the long complex lifetime, coupled with the observed photo-excitation pathway, opens up the possibility to spectroscopically probe the structure of the complex with high resolution, thus elucidating the reaction dynamics.

在许多化学反应中，从反应物到产物的转化是由瞬时中间体配合物介导的。对于涉及具有几个原子的分子的气相反应，这些络合物在离解前通常活度为10 ps或更小，因此很少受外部过程的影响。在这里，我们证明了瞬态中间配合物K ₂ Rb ₂由超冷KRb分子之间的碰撞形成的*在连续波长为1064 nm的连续波激光源的存在下经历了快速的光激发，该波长通常用于限制超冷分子。复合物在超冷条件下的超长寿命有助于这些激发。实际上，通过监测激发光突然去除后复合物种群的变化，我们直接测量了复合物的寿命为360±30 ns，这与我们基于Rice–Ramsperger–Kassel–Marcus的计算（ RRKM）统计理论。我们的结果揭示了在超冷分子实验中广泛观察到的两体损失的起源。此外，复杂的寿命长，再加上观察到的光激发途径，