Book excerpt: The electronic structures of EuNi2Ge2 and GdNi2Ge2 have been investigated using photoemission spectroscopy. The majority of the photoemission intensity near E[subscript F] was due to the mostly Ni 3d states in both materials. CIS spectra were recorded at selected initial-state binding energies (BEs) across the excitation energy range of the Ni 3p threshold in EuNi2Ge2. It is apparent that there is strong Ni d character throughout the valence band from CIS spectra. Resonance measurements of the Eu 4d[Reaction goes right]4f excitations in EuNi2Ge2 reveal that Eu 4f states are localized around 2 eV below E[subscript F]. It is observed that Gd 4f peak in GdNi2Ge2 is more tightly bound by 6.4 eV than in Eu 4f peak in EuNi2Ge2. We have seen a resonant enhancement of the Ni 3d satellite at about 7.4 eV BE, indicating Ni 3d character around the Fermi edge in EuNi2Ge2. The Ni 3d partial spectral weights (PSW) from the Ni 3p[Reaction goes right]3d RPES measurements show that they are very close to the calculated Ni 3d density of states for both materials. We have measured Constant-Final-State (CFS) and Constant-Initial-State (CIS) spectra in an attempt to check the valence of the Eu ion in EuNi2Ge2, and it is verified as 2+. The measured CFS spectra of GdNi2Ge2 was almost the same as those of EuNi2Ge2 and Gd metal, indicating that Gd ions in GdNi2Ge2 are trivalent with the same 4f7 configuration as the Eu ion in EuNi2Ge2. We present experimental energy bands mapped from normal-emission photoelectron spectra of EuNi2Ge2 and GdNi2Ge2 (001) surfaces using synchrotron radiation with increasing photon energies from 14 eV to 54 eV (to 44 eV in the case of GdNi2Ge2). Four and three major photoemission features disperse along the normal [001] Brillouin zone (BZ) direction direction in EuNi2Ge2 and GdNi2Ge2, respectively, in good agreement with the band calculations. We studied quantitatively the effects of band filling on the electronic structures by observing a rigid-band shift of E[subscript F] corresponding simply to an increase of one conduction electron upon going from EuNi2Ge2 to GdNi2Ge2. Segments of the Fermi surfaces (FSs) were mapped by ARPES in the [Gamma]XPZ plane of the Brillouin zones (BZs) for both EuNi2Ge2 and GdNi2Ge2, which are in good agreement with band calculations. We studied how the FS is changed in the [Gamma]XPZ planes of the BZ when one electron is added to EuNi2Ge2, corresponding to GdNi2Ge2, based on the rigid-band approximation. In conclusion, these compounds which give the same Hund's rule ground state (8S--[subscript /]2) were one of suitable systems for the study of effects of band filling on electric structures based on a rigid-band model as a first-order approximation.