In studies investigating the interaction of matter with ultraintense, ultrashort x-ray free electron laser (XFEL) pulses, the evolution kinetics are generally described by directly solving a time-dependent rate equation that considers single-photon and single-electron processes. In the present study, we show the effects of single-photon double photoionization and direct double Auger decay in the K-shell ionization kinetics of XFELs interaction with argon atoms. Because a huge number of coupled transition channels are present in the K-shell ionization, we develop a Monte Carlo method to simulate the complex ionization kinetic processes and give the level population evolution of ions and charge state distribution (CSD). The K-shell-dominated ionization dynamics of Ar irradiated by XFEL pulses with photon energies of 5000, 5500 and 6500 eV are investigated and compared with available experimental observations of the CSD. The results show that the population fractions of Ar⁵⁺, Ar⁶⁺ and Ar⁹⁺ are increased by 78%, 152% and 144%, respectively, by these higher-order processes at a photon energy of 5000 eV. Including the direct double-electron processes, the predicted CSDs are in better agreement with the experiments carried out at the photon energies of 5000, 5500 and 6500 eV. It is expected that the developed theoretical formalism can be used to more accurately calibrate the beam profile and intensity of XFELs.

中文翻译：

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单光子双光电离和直接双俄歇衰变对 Ar 原子与 XFEL 脉冲相互作用的 K 壳电离动力学的影响

在研究物质与超强、超短 X 射线自由电子激光 (XFEL) 脉冲相互作用的研究中，进化动力学通常通过直接求解考虑单光子和单电子过程的时间相关速率方程来描述。在本研究中，我们展示了单光子双光电离和直接双俄歇衰变对 XFEL 与氩原子相互作用的 K 壳层电离动力学的影响。由于 K 壳层电离中存在大量耦合跃迁通道，我们开发了一种蒙特卡罗方法来模拟复杂的电离动力学过程，并给出离子的能级布居数演化和电荷态分布 (CSD)。光子能量为 5000 的 XFEL 脉冲照射的 Ar 的 K 壳层主导电离动力学，研究了 5500 和 6500 eV，并与 CSD 的可用实验观察结果进行了比较。结果表明 Ar 的总体分数在 5000 eV 的光子能量下，通过这些高阶过程，⁵⁺、Ar ⁶⁺和 Ar ^{9+分别增加了 78%、152% 和 144%。}包括直接双电子过程在内，预测的 CSD 与在 5000、5500 和 6500 eV 光子能量下进行的实验更加一致。预计开发的理论形式可用于更准确地校准 XFEL 的光束轮廓和强度。