Study of f-unoccupied electronic states by soft x-ray inverse photoemission

设计并构建了一种新型软x射线逆发射光谱仪。在该光谱仪中，曲率半径为1m的球面光栅没有安装为罗兰安装，而是作为另一种掠入射配置。一个横向曲率半径为1.17 m，纵向曲率半径为0.14 m的环形反射镜将来自样品的发散光聚焦到探测器的光电阴极上。镜面的入射角与镜面法线的夹角为70度。反射镜与探测器的距离为390mm。沿着光路，光栅位于距镜面中心190毫米处。通过与探测器一起旋转光栅支架，我们可以获得软x射线范围内的单色光。通过合理选择参数，可以消去光程方程中的F_1项值，使F_2值最小。当安装2400线/mm的光栅时，可获得20 ~ 140 eV的一阶光谱。典型的分辨率是0.5 eV在20 eV。此外，还设计并制作了一种皮尔斯式电子枪，以产生稳定的电流。CeB_6， CeNi_5的逆4f光发射光谱。用该光谱仪对Ce_7Ni_3进行了测量。在典型的近藤材料CeB_6中，在费米能级以上4.8 eV处观察到未占据的4f_2峰。通过与4f占位谱的比较，估计相关能U_<ff>为6.9 eV。CeNi_5的4f_1和4f_2峰均在费米能级以上1eV和6ev处测量到，而Ce_7Ni_3的4f_2峰仅在费米能级以上观测到，这是由于两种杂化程度不同所致。为了获得与软x射线逆发射光谱的互补信息，测量了铈和铀化合物的x射线光发射光谱、轫致辐射等色线光谱和共振光发射光谱。

We have designed and constructed a new type of an inverse photoemission spectrometer in the soft x-ray range. In this spectrometer, a spherical grating with a radius of curvature 1 m is not installed as Rowland mounting but as another type of a grazing incidence configuration. A toroidal mirror with a radius of curvature 1.17 m in the lateral and 0.14 m in the longitudinal directions focuses the diverging light coming from a sample onto the photocathode of a detector. An incidence angle to the mirror is 70 degrees from the mirror normal. The distance between the mirror and the detector is 390 mm. The grating locates in between at 190 mm apart from the center of the mirror along with the optical path. By rotating the grating holder together with the detector, we can obtain monochmatic light in the soft x-ray range. We can cancel out the F_1 term value and minimaze the F_2 value in the optical path equation by properly selecting parameters. When a grating with 2400 lines/mm is installed, a spectrum in the region from 20 to 140 eV is obtained in the first order. Typical resolution is 0.5 eV at 20 eV.In addition, an electron gun of Pierce type was desgined and fabricated to generate stable current.Inverse 4f photoemission spectra of CeB_6, CeNi_5. and Ce_7Ni_3 have been measured with the use of the present spectrometer. In CeB_6, a typical Kondo materials, unoccupied 4f_2 peak is observed at 4.8 eV above Fermi level. Correlation energy U_<ff> is estimated as 6.9 eV by comparison with the 4f occupied spectrum. Both 4f_1 and 4f_2 peak are measured ato 1eV and 6 eV above Fermi level for CeNi_5, while only a single 4f_2 peak is obsereved for Ce_7Ni_3, originating from the difference of degree of hybridization in both valence-fluctuating materials. In order to get complementary information with soft x-ray inverse photoemission spectra, x-ray photoemission, bremsstrahlung isochromat and resonant photoemission spectra for cerium- and uranium compounds have been measured.