4f relations

Figure 1. X-ray photoemission spectra for 1-2-3 and 2-1-4 copper oxides. The valence bands near the Fermi level are dominated by Cu—O hybrid states. These are not affected by the substitution of rare earth ions, but new 4f related features can be identified by subtraction of the Lai gjSrg 15CUO4 baseline curve (dotted line).

Fig. 4.13, it is seen that only for a few ions the efficient luminescence, related to the 4f" 5d 4f" transition, can be observed. The most promising are the Ce ", Eu ", and ions where the luminescence varied from red to UV depending on the host. In the case of Pr ", UV luminescence is observed. No reports on 4f 5d —> 4f luminescence in are known, probably because the excited D4 state from the 4f electronic configuration is usually located below the lowest state of the 4f 5d electronic configuration. It should also be emphasized that in some materials doped with Ce ", Eu ", and Yb ", 4f 5d 4f -related luminescence may not appear because the lowest state of 4f" 5d electronic configuration is degenerated with the conduction band [67]. 

As an alternative to the 4f electrons, the 4d and especially 3d core levels, with large cross sections for the excitation with laboratory sources such as AIKa radiation (photon energy 1.486 keV), were very appealing to the experimentalist as possible signals for obtaining, in a fast way, a picture of the underlying electronic structure of the lanthanides. This was especially true for Ce, where - without synchrotron radiation - it was clearly very difficult with X-ray Photoemission Spectroscopy (XPS) to detect the 4f-related intensity (Baer et al. 1978). 

At the time of this writing, there is a developing concensus on several of the controversial questions of interpretation for photoelectron spectra of the 4f and valence states of Ce and its compounds. We will address some of these questions in this chapter. Others are discussed elsewhere in this volume or in this series of volumes. It should be stated at the onset that the cerium question is fundamental in solid state physics and this research area is advancing rapidly. As a result, we can expect ever greater understanding of 4f-related phenomena in the years to come. 

In this chapter, we have considered the contributions that have been made by photoelectron spectroscopy to understanding the Ce problem. We have emphasized synchrotron radiation photoemission studies because of the advantages of that technique (section 2). We have shown that the features observed in photoemission spectra are related to the final state rather than the ground state. We have shown that very recent photoemission studies have shown that two 4f related features are observed, and we have discussed how these can be viewed based on the sophisticated models of many authors (sections 3 and 4). Finally, we reviewed the experimental situation for many Ce compounds, noting the significant progress that has been made in the last few years. 

Fig. 30. Spectrum of y-Ceo.gTho.i, recorded at 50 eV photon energy (Mirtensson et al. 1982d). The energies of the 4f related features in CeSb (see text) are marked at 3.05 eV and 0.6 eV. The arrows show how these spectral features in CeSb should be displaced according to the extrapolated Ce 4f shift from fig. 29 (Martensson et al. 1982c).

In fig. 10 we show a BIS spectrum for divalent, semiconducting SmS (Oh and Allen 1984). The BIS spectrum of divalent Sm should be analogous to that of trivalent Eu (4f initial-state configuration). The agreement with calculated f - f multiplet spectra is however poor, and more work is needed to establish experimentally whether all the features seen in the spectrum are 4f related. Oxidation yields Sm2 03 (trivalent Sm). The shape of the BIS spectrum of oxidized SmS is again only in fair agreement with that of trivalent Sm metal. 

Fraction of 4f° related features normalized to total intensity from different spectroscopic experiments. Also given is a crude characterization of the magnetic behaviour, as well as 4f° amplitudes obtained from low-energy experiments the 4f-occupancy % is given by % = 1 —Abbreviations are P = Pauli paramagnetic F = ferromagnetic C — W = Curie-Weiss behaviour N = nonlinear susceptibility over... 

The activity of 4E is regulated in a second way, and this also involves phosphorylation. A recently discovered set of proteins bind to and inactivate 4E. These proteins include 4E-BP1 (BPl, also known as PHAS-1) and the closely related proteins 4E-BP2 and 4E-BP3. BPl binds with high affinity to 4E. The [4E] [BP1] association prevents 4E from binding to 4G (to form 4F). Since this interaction is essential for the binding of 4F to the ribosomal 40S subunit and for correctly positioning this on the capped mRNA, BP-1 effectively inhibits translation initiation. 

An analysis of Eq. (6.13) show that for n = 1 and P = 0.5 and for current densities less than 4% of t the polarization is very low (less than 1 mV) and can practically be neglected. The linear section of the polarization curve extends up to current densities which are 40% of f. At current densities higher than 4f, the semilogarith-mic polarization relation is observed. 

In Part II. Figs. 8 and 11 showed that in Icinthanides the 4f electrons can be described as localized so to be treated, in all respects, as atomic electrons. Thus, for instance, in the lanthanide metcds, properties related to the conduction band (which has essentially a (5d, 6 s) character) may be - in first approximation - separated from the magnetic properties related to the highly localized 4f electrons. 

The related polythiophene 64 has been produced in 63% yield by a Stille coupling from 2-(tributylstannyl)-3,4-(ethylenedioxy)thiophene and 2,6-dibromo-4f/-cyclopenta[2,l- 3,4- ]dithiophen-4-one catalyzed by PdCl2(PPh3)2 or Pd(PPh3)4 in refluxing toluene <2004CM3667>. 

Related to the aforementioned rearrangements are the ring expansions of l,2-dihydro-2-(p -tosyloxymethyl)pyridines in pyridine to 3//-azepines (71JOC978), and of the p-tosyloxy derivative (274) to the unstable 4f/-azepine (275) by solvolysis in acetonitrile (73T391). 

In this section we discuss the properties of an electron in the conduction band of an antiferromagnetic insulator. This may be a simple Mott insulator, but, since the experimental evidence is related to them, we first discuss materials like EuSe, where the europium ion has seven 4f-electrons and electrons can be introduced into the conduction band by doping with GdSe the ion Gd2 + has the same number of f-electrons but one more electron in an outer shell, so a Gd ion acts as a donor. 

---