The light-induced ultrafast switching between molecular isomers norbornadiene and quadricyclane can reversibly store and release a substantial amount of chemical energy. Prior work observed signatures of ultrafast molecular dynamics in both isomers upon ultraviolet excitation but could not follow the electronic relaxation all the way back to the ground state experimentally. Here we study the electronic relaxation of quadricyclane after exciting in the ultraviolet (201 nanometres) using time-resolved gas-phase extreme ultraviolet photoelectron spectroscopy combined with non-adiabatic molecular dynamics simulations. We identify two competing pathways by which electronically excited quadricyclane molecules relax to the electronic ground state. The fast pathway (<100 femtoseconds) is distinguished by effective coupling to valence electronic states, while the slow pathway involves initial motions across Rydberg states and takes several hundred femtoseconds. Both pathways facilitate interconversion between the two isomers, albeit on different timescales, and we predict that the branching ratio of norbornadiene/quadricyclane products immediately after returning to the electronic ground state is approximately 3:2.

光诱导的分子异构体降冰片二烯和四环烷之间的超快转换可以可逆地储存和释放大量的化学能。先前的工作观察到两种异构体在紫外线激发下的超快分子动力学特征，但无法通过实验跟踪电子弛豫一直回到基态。在这里，我们利用时间分辨气相极紫外光电子能谱结合非绝热分子动力学模拟，研究了在紫外线（201纳米）激发后四环烷的电子弛豫。我们确定了电子激发的四环烷分子松弛至电子基态的两种竞争途径。快速路径（<100飞秒）的特点是与价电子态的有效耦合，而慢速路径涉及跨越里德伯态的初始运动，需要数百飞秒。尽管时间尺度不同，这两种途径都有利于两种异构体之间的相互转化，我们预测降冰片二烯/四环烷产物在返回电子基态后立即的支化比约为 3:2。