Chemical reactions are often characterized by their transition state, which defines the critical geometry the molecule must pass through to move from reactants to products. Roaming provides an alternative picture, where in a dissociation reaction, the bond breaking is frustrated and a loosely bound intermediate is formed. Following bond breaking, the two partners are seen to roam around each other at distances of several Ångstroms, forming a loosely bound, and structurally ill-defined, intermediate that can subsequently lead to reactive or unreactive collisions. Here, we present a direct and time-resolved experimental measurement of roaming. By measuring the photoelectron spectrum of UV-excited acetaldehyde with a femtosecond extreme ultraviolet pulse, we captured spectral signatures of all of the key reactive structures, including that of the roaming intermediate. This provided a direct experimental measurement of the roaming process and allowed us to identify the time scales by which the roaming intermediate is formed and removed and the electronic potential surfaces upon which roaming proceeds.

中文翻译：

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漫游中间体及其动力学的直接观察

化学反应通常以其过渡态为特征，过渡态定义了分子从反应物转变为产物必须经过的关键几何形状。漫游提供了另一种情况，在解离反应中，键断裂受到抑制，并形成松散结合的中间体。键断裂后，两个伙伴在几埃的距离内彼此漫游，形成松散结合、结构不明确的中间体，随后可能导致反应性或非反应性碰撞。在这里，我们提出了一种直接的、时间分辨的漫游实验测量方法。通过用飞秒极紫外脉冲测量紫外线激发乙醛的光电子光谱，我们捕获了所有关键反应结构的光谱特征，包括漫游中间体的光谱特征。这提供了漫游过程的直接实验测量，并使我们能够确定漫游中间体形成和去除的时间尺度以及漫游进行的电子势表面。