Green A E, Liu Y, Allum F, Graßl M, Lenzen P, Ashfold M N R, Bhattacharyya S, Cheng X, Centurion M, Crane S W, Forbes R, Goff N A, Huang L, Kaufman B, Kling M-F, Kramer P L, Lam H V S, Larsen K A, Lemons R, Lin M-F, Orr-Ewing A J, Rolles D, Rudenko A, Saha S K, Searles J, Shen X, Weathersby S, Weber P M, Zhao H, Wolf T J A
Stanford PULSE Institute, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA.
EaStCHEM School of Chemistry, University of Edinburgh, Edinburgh EH9 3FJ, United Kingdom.
J Chem Phys. 2025 May 14;162(18). doi: 10.1063/5.0266559.
We investigated the ultrafast structural dynamics of cyclobutanone following photoexcitation at λ = 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 S2 state of cyclobutanone with n3s Rydberg character through its inelastic electron scattering signature with a time constant of (0.29 ± 0.2) ps toward the S1 state. The S1 state population undergoes ring-opening via a Norrish Type-I reaction, likely while passing through a conical intersection with S0. The corresponding structural changes can be tracked by elastic electron scattering signatures. These changes appear with a delay of (0.14 ± 0.05) ps with respect to the initial photoexcitation, which is less than the S2 depopulation time constant. This behavior provides evidence for the ballistic nature of the ring-opening once the S1 state is reached. The resulting biradical species react further within (1.2 ± 0.2) ps via two rival fragmentation channels yielding ketene and ethylene, or propene and carbon monoxide. Our study showcases the value of both 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 with such simulations.
我们使用气相兆电子伏特超快电子衍射技术，研究了环丁酮在λ = 200 nm光激发后的超快结构动力学。我们的研究补充了本期特刊中对同一过程的模拟研究。它通过可清晰分辨的非弹性和弹性电子散射特征，提供了有关电子态布居和结构动力学的信息。我们通过其非弹性电子散射特征，观察到具有n3s里德堡特征的环丁酮光激发S2态的布居数减少，其时间常数为(0.29 ± 0.2) ps，向S1态转变。S1态布居数通过Norrish I型反应发生开环，可能是在通过与S0的锥形交叉点时。相应的结构变化可以通过弹性电子散射特征来追踪。这些变化相对于初始光激发出现延迟(0.14 ± 0.05) ps，这比S2布居数减少的时间常数要小。这种行为为一旦达到S1态开环的弹道性质提供了证据。生成的双自由基物种在(1.2 ± 0.2) ps内通过两个相互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的实验基准的价值，以及在不与此类模拟进行比较的情况下解释此类实验数据的局限性。
