Although collisions between atoms and molecules are largely understood, collisions between two molecules have proven much harder to study. In both experiment and theory, our ability to determine quantum-state-resolved bimolecular cross-sections lags behind their atom–molecule counterparts by decades. For many bimolecular systems, even rules of thumb—much less intuitive understanding—of scattering cross sections are lacking. Here, we report the measurement of state-to-state differential cross sections on the collision of state-selected and velocity-controlled nitric oxide (NO) radicals and oxygen (O₂) molecules. Using velocity map imaging of the scattered NO radicals, the full product-pair correlations of rotational excitation that occurs in both collision partners from individual encounters are revealed. The correlated cross sections show surprisingly good agreement with quantum scattering calculations using ab initio NO−O₂ potential energy surfaces. The observations show that the well-known energy-gap law that governs atom–molecule collisions does not generally apply to bimolecular excitation processes, and reveal a propensity rule for the vector correlation of product angular momenta.

尽管人们对原子和分子之间的碰撞已广为人知，但事实证明，两个分子之间的碰撞更难研究。在实验和理论上，我们确定量子态分辨的双分子截面的能力都落后于其原子-分子对应物数十年。对于许多双分子系统，甚至缺乏散射横截面的经验法则（远没有那么直观的理解）。在这里，我们报告在状态选择和速度控制的一氧化氮（NO）自由基与氧气（O ₂）分子。使用散布的NO自由基的速度图成像，揭示了两次碰撞伙伴在两次碰撞中发生的旋转激发的完整产物对相关性。相关的横截面显示出与使用从头开始的NO-O ₂势能面的量子散射计算出令人惊讶的良好一致性。观测结果表明，支配原子-分子碰撞的众所周知的能隙定律通常不适用于双分子激发过程，并且揭示了产物角动量矢量相关性的倾向性规则。