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

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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.