Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany.
J Chem Phys. 2012 Jun 7;136(21):214310. doi: 10.1063/1.4722756.
We devise a theoretical description for the response of nitrogen molecules (N(2)) to ultrashort and intense x rays from the free electron laser Linac Coherent Light Source (LCLS). We set out from a rate-equation description for the x-ray absorption by a nitrogen atom. The equations are formulated using all one-x-ray-photon absorption cross sections and the Auger and radiative decay widths of multiply-ionized nitrogen atoms. Cross sections are obtained with a one-electron theory and decay widths are determined from ab initio computations using the Dirac-Hartree-Slater (DHS) method. We also calculate all binding and transition energies of nitrogen atoms in all charge states with the DHS method as the difference of two self-consistent field (SCF) calculations (ΔSCF method). To describe the interaction with N(2), a detailed investigation of intense x-ray-induced ionization and molecular fragmentation are carried out. As a figure of merit, we calculate ion yields and the average charge state measured in recent experiments at the LCLS. We use a series of phenomenological models of increasing sophistication to unravel the mechanisms of the interaction of x rays with N(2): a single atom, a symmetric-sharing model, and a fragmentation-matrix model are developed. The role of the formation and decay of single and double core holes, the metastable states of N(2)(2+), and molecular fragmentation are explained.
我们为氮分子（N2）对自由电子激光直线加速器相干光源（LCLS）的超短强 X 射线的响应设计了一种理论描述。我们从一个描述氮原子对 X 射线吸收的速率方程开始。这些方程使用单 X 射线光子吸收截面和多电离氮原子的俄歇和辐射衰减宽度来制定。截面是用单电子理论获得的，衰减宽度是使用从头算计算（使用 Dirac-Hartree-Slater（DHS）方法）确定的。我们还使用 DHS 方法（ΔSCF 方法）作为两个自洽场（SCF）计算的差异，计算了所有电荷态氮原子的所有结合能和跃迁能。为了描述与 N2 的相互作用，我们对强 X 射线诱导的电离和分子碎裂进行了详细的研究。作为一个衡量标准，我们计算了 LCLS 最近实验中测量的离子产率和平均电荷态。我们使用一系列越来越复杂的唯象模型来揭示 X 射线与 N2 相互作用的机制：开发了一个单原子模型、一个对称共享模型和一个碎裂矩阵模型。解释了单和双芯空位的形成和衰减、N2（2+）的亚稳态以及分子碎裂的作用。
