近日，德国欧洲X射线自由电子激光装置（XFEL）的Laurent Mercadier与德国自由电子激光科学研究中心的Andrei Benediktovitch等人合作并取得一项新进展。他们对X射线自由电子激光器所产生热致密物质的瞬态吸收进行了研究。相关研究成果已于2024年7月29日在国际知名学术期刊《自然—物理学》上发表。
该研究团队利用强超快X射线自由电子激光脉冲，在大辐照强度范围内，通过L边缘X射线吸收光谱技术，同时创建了温暖的致密铜并对其进行了表征。当脉冲强度低于1015 W/cm-2时，L边缘以下出现了一个吸收峰，这是由3d波段的瞬态耗尽所引起的。随着强度的增加，该吸收峰向较低能量方向移动，这表明在强X射线激发下3d波段发生了动态变化。
在更高强度下，由于大量电离和碰撞，X射线自由电子激光脉冲从反向饱和吸收转变为饱和吸收。这两种非线性效应有望在X射线脉冲整形中得到应用。为了解释这些观测结果，研究人员采用了理论计算，将基于运动玻尔兹曼方程的模型与有限温度实空间密度泛函理论相结合。该研究成果可用于构建热致密物质中电子结构的非平衡模型。
据悉，热致密物质位于等离子体和凝聚态之间，在天体物理学、行星科学和惯性约束聚变研究中发挥着重要作用。然而，在强激光脉冲照射下，其电子结构和离子结构尚不清楚。
附：英文原文
Title: Transient absorption of warm dense matter created by an X-ray free-electron laser
Author: Mercadier, Laurent, Benediktovitch, Andrei, Krui, pela, Kas, Joshua J., Schlappa, Justine, Agker, Marcus, Carley, Robert, Fazio, Giuseppe, Gerasimova, Natalia, Kim, Young Yong, Le Guyader, Loc, Mercurio, Giuseppe, Parchenko, Sergii, Rehr, John J., Rubensson, Jan-Erik, Serkez, Svitozar, Stransky, Michal, Teichmann, Martin, Yin, Zhong, itnik, Matja, Scherz, Andreas, Ziaja, Beata, Rohringer, Nina
Issue&Volume: 2024-07-29
Abstract: Warm dense matter is at the boundary between a plasma and a condensed phase and plays a role in astrophysics, planetary science and inertial confinement fusion research. However, its electronic structure and ionic structure upon irradiation with strong laser pulses remain poorly understood. Here, we use an intense and ultrafast X-ray free-electron laser pulse to simultaneously create and characterize warm dense copper using L-edge X-ray absorption spectroscopy over a large irradiation intensity range. Below a pulse intensity of 1015Wcm-2, an absorption peak below the L edge appears, originating from transient depletion of the 3d band. This peak shifts to lower energy with increasing intensity, indicating the movement of the 3d band upon strong X-ray excitation. At higher intensities, substantial ionization and collisions lead to the transition from reverse saturable absorption to saturable absorption of the X-ray free-electron laser pulse, two nonlinear effects that hold promise for X-ray pulse-shaping. We employ theoretical calculations that combine a model based on kinetic Boltzmann equations with finite-temperature real-space density-functional theory to interpret these observations. The results can be used to benchmark non-equilibrium models of electronic structure in warm dense matter.
DOI: 10.1038/s41567-024-02587-w
Source: https://www.nature.com/articles/s41567-024-02587-w