4f-level

Cerium is especially intereshng because of its variable electronic structure. The energy of the inner 4f level is nearly the same as that of the outer or valence electrons, and only small amounts of energy are required to change the relahve occupancy of these electronic levels. This gives rise to dual valency states. 

Very important final state effects are those which are described in the final state multiplet theory. They occur in cores containing unsaturated, localized shells (open-shell cores). The theory has been successfully apphed, e.g., in the interpretation of the spectral response from the 4f levels of lanthanides . ... 

In a recent investigation31 of TaC(lll) the surface shifted Ta 4f levels were investigated before and after depositing a monolayer of graphite on the surface. A dense carbon overlayer on the polar TaC(lll) surface might be expected to quench the surface Ta core level and to leave only a bulklike core level. The spectrum recorded after deposition of a graphite monolayer, however, showed an attenuated and shifted surface Ta core level, shifted about 0.3 eV closer to the bulk peak. It thus appears that not even a dense graphite monolayer is sufficient to produce a bulk-like electronic environment for the surface Ta atoms on TaC(lll). 

There is an interesting analogy between the anomalous behavior of Pr in boro-carbides with the well-known anomalous properties of Pr-containing cuprates (Lynn, 1997, see Section 1.3). For PrBa2Cu30 -, e.g., it is widely accepted that the absence of superconductivity and the anomalously high Tn are connected with the increased hybridization of 4f levels with planar oxygen-derived states being important for superconductivity of doped holes. 

Similarly, peak Bi corresponds to transitions to the (4f5/2)(5d ,) configuration, while peak B2 corresponds to transitions to the (4f7/2)(5d ,) configuration. These results also indicate that the splitting between peak Bi and B2 originates from the spin-orbit splitting of Pr 4f levels. Compared with peaks A and B, the compositions of peaks C and E are complicated. Peak C corresponds to transitions to the mixed states of the (4f5/2)(5dc), (4f7/2)(5dc), (4f5/2)(5d ), and (4f7/2)(5drf) configurations which simply corresponds to the transitions from 4f states to... 

The theoretical spectra for the three relaxed models are shown in fig. 24. As the bond lengths increase, the separations of the peaks decrease. The theoretical multiplet energy levels for these three models are compared in fig. 25. As shown in the previous section, the separation between peak a and peaks 02, <23 originates from the spin-oibit splittings of the Pr 4f levels which are distinguished by red and blue lines. The origin of the separation between peaks <24,... 

The iminocryptate 24, which can be prepared by a one-pot template reaction, was initially designed for the simultaneous complexation of two 4f ions (Platas et al., 2000) or one 4f ion and one 3d (e.g. Zn11, Cu11) ion (Rodriguez-Cortinas et al., 2002). Sensitization of Ndm and Ybm was also observed, particularly in the 4f-Znn heterodimetallic cryptates, for which the crystal field splitting of the 4f levels could be interpreted in terms of a distorted C3 local symmetry. 

B. Johansson and N. Martensson, Thermodynamic aspects of 4f levels in metals and compounds 361... 

Fig. 2 Valence band photoemission profiles of empty C82 and Gd C82, recorded at room temperature with He Ia (21.22 eV) radiation. The inset shows the region close to the Fermi level on an expanded scale. For this photon energy, the photoionisation cross sections of the C 2s and C 2p levels dominate that of the Gd 4f levels...

Photoemission from the 4f levels of the rare-earth metals and their compounds provides a clear example of a situation in which the N-1 nature of the photoemission final state needs to be explicitly taken into account to give a correct interpretation of the data. 

In the Tm case, resonant photoemission proves that there is no significant hybridisation between the rare-earth 5d levels and the carbon electronic states thus the Tm ions have an essentially ionic interaction with their fullerene host. In the Gd case, the magnitude of the resonant enhancement of emission from the 4f levels signals the presence of hybridisation between the Gd valence levels and those of the carbon cage. However, this does not alter the conclusion as regards the Gd s trivalency and the transfer of essentially three electrons to the fullerene MOs. 

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