利用飞秒X射线/X射线泵浦探测光谱研究双原子分子中超快X射线诱导的核动力学
Ultrafast x-ray-induced nuclear dynamics in diatomic molecules using femtosecond x-ray/x-ray pump-probe spectroscopy
摘要：
产生具有受控时间延迟的两个强烈的飞秒X射线脉冲的能力开启了对X射线引起的现象进行时间分辨实验的可能性。我们已经应用这种能力来研究双原子分子中的光诱导动力学。在由低Z元素组成的分子中，\textit{K} -壳层电离产生核-空穴态，其中主要衰变模式是涉及价层中的两个电子的俄歇过程。俄歇衰变后，瞬态二价空穴态的核波包在飞秒时间尺度上继续演化，导致分离的原子离子或长寿命的准束缚态。通过使用X射线泵和X射线探测脉冲调谐以上的\textit{K}壳层电离阈值的氮分子，我们能够观察离子解离的进展，通过测量随时间变化的动能释放不同的分裂通道。我们模拟的测量N $_2$与分子动力学模型，占\textit{K}壳层电离，俄歇衰变，和时间演化的核波包。模拟结果不仅解释了离解态释放动能分布随时间变化的特征，还揭示了准束缚态的贡献。
英文摘要：
The capability of generating two intense, femtosecond x-ray pulses with controlled time delay opens the possibility of performing time-resolved experiments for x-ray induced phenomena. We have applied this capability to study the photoinduced dynamics in diatomic molecules. In molecules composed of low-Z elements, \textit{K}-shell ionization creates a core-hole state in which the main decay mode is an Auger process involving two electrons in the valence shell. After Auger decay, the nuclear wavepackets of the transient two-valence-hole states continue evolving on the femtosecond timescale, leading either to separated atomic ions or long-lived quasi-bound states. By using an x-ray pump and an x-ray probe pulse tuned above the \textit{K}-shell ionization threshold of the nitrogen molecule, we are able to observe ion dissociation in progress by measuring the time-dependent kinetic energy releases of different breakup channels. We simulated the measurements on N$_2$ with a molecular dynamics model that accounts for \textit{K}-shell ionization, Auger decay, and the time evolution of the nuclear wavepackets. In addition to explaining the time-dependent feature in the measured kinetic energy release distributions from the dissociative states, the simulation also reveals the contributions of quasi-bound states.
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