Modern free-electron lasers (FEL) operating in XUV (extreme ultraviolet) or X-ray range allow an access to novel research areas. An example is the ultrafast ionization of a solid by an intense femtosecond FEL pulse in XUV which consequently leads to a change of the complex index of refraction on an ultrashort timescale. The photoionization and subsequent impact ionization resulting in electronic and atomic dynamics are modeled with our hybrid code XTANT(X-ray thermal and non-thermal transitions) and a Monte Carlo code XCASCADE(X-ray-induced electron cascades). The simulations predict the temporal kinetics of FEL-induced electron cascades and thus yield temporally and spatially resolved information on the induced changes of the optical properties. In a series of experiments at FERMI and LCLS, single shot measurements with spatio-temporal encoding of the ionization process have been performed by a correlation of the FEL pump pulse with an optical femtosecond probe pulse. An excellent agreement between the experiment and the simulation has been found. We also show that such kind of experiments forms the basis for pulse duration and arrival time jitter monitoring as currently under development for XUV-FELs.

中文翻译：

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使用强超短 XUV 和 X 射线自由电子激光脉冲进行时间分辨电离测量

在 XUV（极紫外）或 X 射线范围内运行的现代自由电子激光器 (FEL) 允许进入新的研究领域。一个例子是在 XUV 中通过强烈的飞秒 FEL 脉冲对固体进行超快电离，从而导致复合折射率在超短时间尺度上发生变化。使用我们的混合代码 XTANT（X 射线热和非热跃迁）和蒙特卡罗代码 XCASCADE（X 射线诱导电子级联）对导致电子和原子动力学的光电离和随后的碰撞电离进行建模。模拟预测了 FEL 诱导的电子级联的时间动力学，从而产生了关于光学性质的诱导变化的时间和空间解析信息。在 FERMI 和 LCLS 的一系列实验中，已通过 FEL 泵浦脉冲与光学飞秒探测脉冲的相关性进行了具有电离过程时空编码的单次测量。已经发现实验和模拟之间非常吻合。我们还表明，此类实验构成了目前正在为 XUV-FEL 开发的脉冲持续时间和到达时间抖动监测的基础。