The photoelectron angular distribution asymmetry parameters β of the Xe 3d subshell were investigated using an x-ray free-electron laser (XFEL) at photon energies of 750 and 800eV. Owing to the perfect polarization of the XFEL and two-dimensional momentum imaging capability of our velocity map imaging spectrometer, we determined the β values with high accuracy. The β values were also investigated based on relativistic time-dependent density functional theory calculations of up to 900eV of photon energies. By comparing all the available experimental results including our data with the most reliable theories on the photon energy dependence of the β parameters, serious differences are noted between the experiments and theories. Further studies on resolving this difference will provide new insight into the photoionization processes of the deep inner shells.

使用X射线自由电子激光器（XFEL）在750和800eV的光子能量下研究了Xe 3d子壳的光电子角分布不对称参数β。由于 XFEL 的完美极化和我们速度图成像光谱仪的二维动量成像能力，我们高精度地确定了β值。该β值是基于高达900eV光子能量的相对论时间依赖密度泛函理论计算了研究。通过将所有可用的实验结果（包括我们的数据）与关于β光子能量依赖性的最可靠理论进行比较参数，注意到实验和理论之间存在严重差异。解决这种差异的进一步研究将为深入内壳的光电离过程提供新的见解。