Crystal-field probe

These factors are relatively invariable with regard to k. Equation (62) can be considered as a geometrical rotational-invariant average for the maximum crystal-field splitting in any point symmetry. This approximation is good if the D are stationary in k, i.e. if the Z7, IT and if have nearly the same value (or at least the same order of magnitude). Based on the stationarity of the squares of the reduced matrix elements, Auzel (Auzel and Malta 1983) has selected for the lanthanide ions levels which should theoretically provide the best crystal-field probe H4 and for Pr ", I13/2 and I9/2 forNd ", I13/2 and Sr,5/2 for Sm +, Fj for Eu, Fj for Tb +, %n, %U2, %V2, His/2 and... 

The best choice for a crystal-field probe" is the Eu ion, for the following reasons ... 

The rare earth elements are different from other elements because the optical transitions between levels of the fn configuration are inherently very sharp-lined and have well-resolved structure characteristic of the local crystal fields around the ion. In minerals, this characteristic provides an excellent probe of the local structure at the atomic level. Examples will be shown from our work of how site selective laser spectroscopy can be used to determine the thermal history of a sample, the point defect equilibria that are important, the presence of coupled ion substitution, the determination of multiple phases, and stoichiometry of the phase. The paper will also emphasize the fact that the usefulness and the interpretation of the rare earth luminescence is complicated by the presence of quenching and disorder in mineral samples. One in fact needs to know a great deal about a sample before the wealth of information contained in the site selective luminescence spectrum can be understood. 

Crystal Field Splittings. The key idea is to use fluorescent probe ions that have spectra that are sensitive to the crystal fields they experience within a material (2). Such ions are generally ones with unfilled inner orbitals such as lanthanides (e.g. Pr3+, Eu3+, Er +), actinides (e.g. U), or transition metals (e.g. Cr3+, Re4+, Os4+). 

The d electrons within the dn electron configuration of the transition metals have much stronger interactions with the crystal field so crystal field interactions are comparable to the interactions within the atom. The spectral transitions can change more drastically to reflect the changes in the crystal field. The line-widths of the transitions are also much broader. Not all transition metals are useful as probe ions because the lines are too broad to allow one to resolve features in samples where there are multiple local environments about the ion. In fact, only some atomic states of the probe ions are useful because most states interact too strongly with the crystal field to give narrow enough lines. 

The 5fn electrons of the actinides represent an intermediate case where there is still shielding of the crystal fields but it is not as effective as in the lanthanides. The crystal field interactions are larger than the lanthanides but not as large as in the transition metals. The lines of most transitions are sharp and all the actinide ions could be used potentially as probes of the local environments of minerals. 

Photoluminescence excitation spectroscopy (PLE) was performed on GaN Er by several groups [15-17], FIGURE 2 shows typical PLE spectra probing the second and third excited Er states while monitoring the 1.539 pm PL line. The Stark splitting caused by the crystal field is clearly shown. The excitation source was 250 mW from a tunable Ti sapphire laser. 

Kinetics and mechanisms complex formation involving rate expressions, rate laws, dissociative and associative pathways, techniques used in probing reaction mechanisms, crystal field effects are discussed in the following chapter. 

Equation (10) shows that the isomer shift IS is a direct measure of the total electronic density at the probe nucleus. This density derives almost exclusively from 5-type orbitals, which have non-zero electron densities at the nucleus. Band electrons, which have non-zero occurrence probabilities at the nucleus and 5-type conduction electrons in metals may also contribute, but to a lesser extent. Figure 3 shows the linear correlation that is observed between the experimental values of Sb Mossbauer isomer shift and the calculated values of the valence electron density at the nucleus p (0). The total electron density at the nucleus p C ) (Eq. 10) is the sum of the valence electron density p (0) and the core electron density p (0), which is assumed to be constant. This density is not only determined by the 5-electrons themselves but also by the screening by other outer electrons p-, d-, or /-electrons) and consequently by the ionicity or covalency and length of the chemical bonds. IS is thus a probe of the formal oxidation state of the isotope under investigation and of the crystal field around it (high- and low-spin Fe may be differentiated). The variation of IS with temperature can be used to determine the Debye temperature of a compound (see Eq. (13)). 

Cr +) have been probed by absorption and luminescence spectroscopy. The strongest interaction occurs for the NF complex, where the lowest energy singlet state is very close in energy to a spin allowed crystal field band, giving rise to intense vibronic patterns. ... 

Other probe molecules used for the EPR study of Ce02-based systems are NO and nitroxyl radicals [31, 32, 33] in this case, the neutral molecules themselves are paramagnetic, and give information without electron transfer. In the case of adsorbed NO, gz values are sensitive to the crystal field strength at adsorption sites of oxides in exactly the same way as in the case of O2 NO has been thus used to study the... 

For all the above reasons, ruthenium has become a very popular redox site as a (relatively cheap) heavy metal, its complexes are kinetically stable and its crystal field A parameter value is relatively high. Hence, for most common complexes of ruthenium(II) of pseudo-octahedral geometry, the d electronic configuration is a low-spin diamagnetic state, and frontier orbitals, involved in the redox processes are essentially of the nonbonding tag type. Eventually its rich photochemistry may be used to probe excited-state behaviors. 

In addition the crystal field is responsible for the splitting of certain optical transitions. Obvious is the following statement different host lattices —different crystal lelds different splittings. In this way the optical center can serve as a probe of he surroundings observed splittings yield the symmetry of the site. 

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