A reduced density matrix (RDM) formalism to simulate the dynamics of core-valence electronic excitation in a metal induced by a femtosecond x-ray laser pulse is presented. The theory is based on the time-dependent unrestricted Hartree–Fock (TDUHF) equation for a one-electron RDM combined with an off-diagonal collision term in the Born approximation (BA), where molecular orbitals (MOs) of a model cluster are used as a basis set. These MOs are calculated with a newly developed semiempirical method in the neglect of diatomic differential overlap approximation that takes into account atomic core structures and valence-orbital hybridization. The off-diagonal component of core-valence RDM is decomposed into a rapidly oscillating factor synchronizing with the x-ray field and a slowly varying envelope; time evolution of the latter yields induced electric polarization and complex electric susceptibility. As a numerical test, K -shell absorption near-edge spectra of copper are compu...

中文翻译：

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飞秒X射线吸收及其激子增强的密度矩阵形式论

提出了一种简化密度矩阵（RDM）形式，以模拟飞秒X射线激光脉冲在金属中引起的核心价电子激发的动力学。该理论基于与时间有关的不受限制的Hartree-Fock（TDUHF）方程，该方程用于在Born近似（BA）中结合非对角碰撞项的单电子RDM，其中模型簇的分子轨道（MOs）为用作基础集。这些MOs是用新开发的半经验方法计算的，而忽略了考虑原子核结构和价-轨杂化的双原子微分重叠近似。核心价RDM的非对角线成分分解为与X射线场同步的快速振荡因子和缓慢变化的包络；后者的时间演化产生感应的极化和复杂的电化率。作为数值测试，计算了铜的K壳吸收近边缘光谱。