In order to clarify bulk 4f electronic states of a Kondo semiconductor CeNiSn, we have measured Ce 3d-4f resonance photoemission (RPES) spectra of CeNiSn and its reference material CePdSn with unprecedented high resolution (100 meV at the photon energies of about 880 eV) at a twin-helical undulator beamline BL25SU in SPring-8 [1]. The Ce 3d-4f RPES mainly reflects bulk 4f states [2] due to a longer photoelectron mean free path compared with the 4d-4f RPES. The obtained 3d-4f on-resonance spectra show a clear doublet structure consisting of a tail of the Kondo peak (f_5/2¹ final state) and its spin-orbit partner (f_7/2¹ final state) for both materials. The intensity of the f_5/2¹ final state is stronger than that of the f_7/2¹ final state in the 3d-4f RPES spectra of CeNiSn while the f_5/2¹ final state is weak for CePdSn. Both 3d-4f and "surface-sensitive" 4d-4f RPES experimental spectra are well fitted by the spectral calculations based on a single impurity Anderson model with considering the bulk and less hybridized surface components. The calculation also gives a reasonable value of the Kondo temperature for CeNiSn. The 3d-4f off-resonance spectra ("non-4f" spectra) are properly reproduced by band-structure calculations for both compounds.

[1] Y. Saitoh et al., Synchrotron Rad., vol. 5, p. 542 (1998).
[2] A. Sekiyama et al., Nature, vol.403, p. 396 (2000).

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