Photoinduced isomerization reactions lie at the heart of many chemical processes in nature. The mechanisms of such reactions are determined by a delicate interplay of coupled electronic and nuclear dynamics occurring on the femtosecond scale, followed by the slower redistribution of energy into different vibrational degrees of freedom. Here we apply time-resolved photoelectron spectroscopy with a seeded extreme ultraviolet free-electron laser to trace the ultrafast ring opening of gas-phase thiophenone molecules following ultraviolet photoexcitation. When combined with ab initio electronic structure and molecular dynamics calculations of the excited- and ground-state molecules, the results provide insights into both the electronic and nuclear dynamics of this fundamental class of reactions. The initial ring opening and non-adiabatic coupling to the electronic ground state are shown to be driven by ballistic S–C bond extension and to be complete within 350 fs. Theory and experiment also enable visualization of the rich ground-state dynamics that involve the formation of, and interconversion between, ring-opened isomers and the cyclic structure, as well as fragmentation over much longer timescales.

光诱导的异构化反应是自然界许多化学过程的核心。此类反应的机制由飞秒级上发生的耦合的电子和核动力学的微妙相互作用决定，然后再将能量重新缓慢分配到不同的振动自由度中。在这里，我们应用时间分辨光电子能谱与种子极紫外自由电子激光，以追踪紫外光激发后气相噻吩分子的超快开环。当与激发态和基态分子的从头算电子结构和分子动力学计算相结合时，结果可提供对该基本反应类别的电子动力学和核动力学的深刻见解。最初的开环和与电子基态的非绝热耦合被证明是由弹道S–C键扩展驱动的，并在350 fs内完成。理论和实验还可以使丰富的基态动力学可视化，这些动力学涉及开环异构体和环状结构的形成以及它们之间的相互转化，以及在更长的时间范围内的碎片化。