Joint Attosecond Science Laboratory, National Research Council of Canada and University of Ottawa, 100 Sussex Drive, Ottawa, Ontario K1A 0R6, Canada.
Department of Physics, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada.
J Chem Phys. 2019 Nov 7;151(17):174301. doi: 10.1063/1.5095430.
We study the near-threshold photodissociation dynamics of NO by a kinematically complete femtosecond pump-probe scheme using a cold target recoil ion momentum spectrometer. We excite NO to the optically bright ÃB state with a 400 nm pulse and probe the ensuing dynamics via strong field single and double ionization with a 25 fs, 800 nm pulse. The pump spectrum spans the NO(XΠ) + O(P) dissociation channel threshold, and therefore, following internal conversion, excited NO is energetically prepared both "above threshold" (dissociating) and "below threshold" (nondissociating). Experimentally, we can clearly discriminate a weak two-photon pump channel from the dominant single-photon data. In the single ionization channel, we observe NO fragments with nonzero momentum at 200 fs delay and an increasing yield of NO fragments with near-zero momentum at 3.0 ps delay. For double ionization events, we observe a time-varying Coulombic kinetic energy release between the NO and O fragments impulsively created from the evolving "hot" neutral ground state. Supported by classical trajectory calculations, we assign the decreasing Coulombic kinetic energy release at longer time delays to the increasing average NO-O distances in the ground electronic state during its large amplitude phase space evolution toward free products. The time-resolved kinetic energy release in the double ionization channel probes the large amplitude ground state evolution from a strongly coupled "inner region" to a loosely coupled "outer region" where one O atom is on average much further away from the NO. Both the time evolution of the kinetic energy release and the NO angular distributions support our assignments.
我们通过使用冷靶反冲离子动量谱仪的动力学完整飞秒泵浦-探测方案研究了 NO 的近阈光解动力学。我们用 400nm 的脉冲将 NO 激发到光学明亮的 ÃB 态，并用 25fs、800nm 的脉冲探测随后的动力学，包括强场单电离和双电离。泵浦光谱跨越了 NO(XΠ)+O(P) 离解通道的阈值，因此，在内部转化之后，激发的 NO 在能量上既可以“高于阈值”（离解），也可以“低于阈值”（不离解）。实验上，我们可以清楚地从主要的单光子数据中分辨出较弱的双光子泵浦通道。在单电离通道中，我们在 200fs 延迟时观察到具有非零动量的 NO 碎片，并在 3.0ps 延迟时观察到具有近零动量的 NO 碎片的产率增加。对于双电离事件，我们观察到在不断演化的“热”中性基态中，从瞬态创建的 NO 和 O 碎片之间的时变库仑动能释放。通过经典轨迹计算，我们将较长延迟时库仑动能释放的减小归因于在其向自由产物的大振幅相空间演化过程中，基态中平均 NO-O 距离的增加。双电离通道中的时间分辨动能释放探测了从强耦合的“内区”到弱耦合的“外区”的大振幅基态演化，其中一个 O 原子平均离 NO 更远。动能释放的时间演化和 NO 的角分布都支持我们的分配。
