标题： 使用超快电子衍射成像环丁酮的光化学反应：实验结果 显示英文标题

标题： Imaging the Photochemistry of Cyclobutanone using Ultrafast Electron Diffraction: Experimental Results

摘要： 我们使用气相兆电子伏特超快电子衍射研究了在$\lambda=200$ nm 处光激发后环丁酮的超快结构动力学。 我们的研究补充了本期特刊中对同一过程的模拟研究。 通过可良好分离的非弹性与弹性电子散射特征，它提供了关于电子态布居和结构动力学的信息。 我们观察到环丁酮的具有 n3s 赖德伯特征的光激发 S$_2$态通过其非弹性电子散射特征，在 $(0.29 \pm 0.2)$ ps 的时间常数下向 S$_1$态消退。 S$_1$态的布居通过诺里什 I 型反应发生环开裂，可能是在经过与 S$_0$态的锥形交叉点时发生的。 相应的结构变化可以通过弹性电子散射特征进行跟踪。 这些变化相对于初始光激发有$(0.14 \pm 0.05)$皮秒的延迟，这小于 S$_2$的消布时间常数。 这种行为为达到 S$_1$状态后环打开的弹道性质提供了证据。 产生的双自由基物种在$(1.2 \pm 0.2)$皮秒内通过两种竞争的碎片化通道进一步反应，生成乙烯酮和乙烯，或丙烯和一氧化碳。 我们的研究展示了气相超快衍射研究作为非绝热动力学模拟方法实验基准的价值，以及在没有与此类模拟进行比较的情况下对这类实验数据解释的局限性。 显示英文摘要

摘要： We investigated the ultrafast structural dynamics of cyclobutanone following photoexcitation at $\lambda=200$ nm using gas-phase megaelectronvolt ultrafast electron diffraction. Our investigation complements the simulation studies of the same process within this special issue. It provides information about both electronic state population and structural dynamics through well-separable inelastic and elastic electron scattering signatures. We observe the depopulation of the photoexcited S$_2$ state of cyclobutanone with n3s Rydberg character through its inelastic electron scattering signature with a time constant of $(0.29 \pm 0.2)$ ps towards the S$_1$ state. The S$_1$ state population undergoes ring-opening via a Norrish Type-I reaction, likely while passing through a conical intersection with S$_0$. The corresponding structural changes can be tracked by elastic electron scattering signatures. These changes appear with a delay of $(0.14 \pm 0.05)$ ps with respect the initial photoexcitation, which is less than the S$_2$ depopulation time constant. This behavior provides evidence for the ballistic nature of the ring-opening once the S$_1$ state is reached. The resulting biradical species react further within $(1.2 \pm 0.2)$ ps via two rival fragmentation channels yielding ketene and ethylene, or propene and carbon monoxide. Our study showcases both the value of gas-phase ultrafast diffraction studies as an experimental benchmark for nonadiabatic dynamics simulation methods and the limits in the interpretation of such experimental data without comparison to such simulations.