3d—4f linewidths

We see that the values of 3d-4f linewidths of normal R metals and oxides obtained either (i) by neglecting the instrumental broadening effects or (ii) by overestimating these and (iii) invoking line saturation effects differ considerably. Also, some doubt persists as to whether 3d-4f linewidths of R are really influenced by the existence, in soUds, of further decay paths for the 3d hole in the presence of an excited 4f electron. From the experimental standpoint, this situation is further complicated if we consider an observation under the same resolution of the narrower 3d-4f lines in R fluorides (Kamatak et al. 1981) and unusually broad lines or multiplets in R dioxides (Kamatak et al. 1987a,b). Thus, it became cmcial to know whether, in certain chemical environments, the change observed in the 3d-4f line or multiplet widths is a real one. For this reason a quantitative estimate of instrumental and other broadening effects became necessary. 

In the following paragraphs we discuss photoabsorption features of atoms, clusters and solids of some of the Sm group elements, whose 3d-4f linewidths exhibit normal behaviour in their different valence phases. This will lead us to invoke a new mechanism involving 4f-covalence to account for the unusually broad 3d-4f lines observed for Ce group dioxides. 

We next discuss the case of Ce, Pr and Tb oxides in which abnormally large 3d-4f mul-tiplet linewidths are observed. These elements belong to the second group of sect. 2.9. 

Esteva and Kamatak (1987) proposed a semi-quantitative estimate of line saturation effects usually encountered in absorption measurements by transmission and photoyield methods. The estimate of line saturation was obtained by conqiaring the 3d linewidth of La obtained by photo yield and by resonant photon scattering. Some quantitative information on the line saturation effect was obtained fi om resonant photon scattering experiments. As opposed to the election yield method, in the latter experiments, the scattered photon is detected as a function of incident photon energy. Spectra are scaimed around the 3ds/2 and 3d3/2 resonance absorption lines. The process involved in the scattering is complementary to photoexcitation, in which remission of a photon due to de-excitation of a 4f electron occurs. 

In metals 4f ions are much better ESR probes than 3d ions due to fact that the exchange parameter Jie is about ten times smaller for the exchange interactions between 4f electrons and the band states. The solubility of the 4f ions in many metallic elements is sufficient to perform ESR experiments and at least at low temperatures, narrow absorption lines can be expected. In most cases line positions and linewidths are determined by CF effects in addition to the well-known Korringa behavior. Again the ions with an S ground state like Gd " and Eu " are the preferred probes for ESR investigations. They warrant small crystalline-field influences on the linewidth and on the resonance field. This is the reason that by far most of the publications deal with Gd ". ... 

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