Bulk 4f electronic states of Ce-based heavy fermion system probed by high-resolution resonance photoemission

"Bulk-sensitive" Ce 3d - 4f resonance photoemission (RPES) with an unpreced entedly high resolution has revealed a clear difference of bulk Ce 4f spect ral weights of heavy fermion system CeRu2Si2 and CeRu2Ge2 in the region of the tail of the Kondo peak and its spin-orbit partner. The significant spec tral difference in both materials is in strong contrast to the mutually sim ilar "surface-sensitive" 4d - 4f RPES. The obtained bulk-sensitive spectra are well reproduced by a single impurity model. Consideration of the crysta lline electric field splitting is important for estimating a realistic Kond o temperature from the bulk 4f spectra.