Complexes emission spectra from transition

Since an atom of a given element gives rise to a definite, characteristic line spectrum, it follows that there are different excitation states associated with different elements. The consequent emission spectra involve not only transitions from excited states to the ground state, e.g. E3 to E0, E2 to E0 (indicated by the full lines in Fig. 21.2), but also transisions such as E3 to E2, E3 to 1( etc. (indicated by the broken lines). Thus it follows that the emission spectrum of a given element may be quite complex. In theory it is also possible for absorption of radiation by already excited states to occur, e.g. E, to 2, E2 to E3, etc., but in practice the ratio of excited to ground state atoms is extremely small,... 

The absorption spectrum of the gold-lead complex shows two bands at 290 nm (e = 28598 M-1cm-1) and 385 nm (e = 7626 M-1cm-1), while the emission spectrum in the solid state shows only one band at 752 nm at room temperature. It was assigned to a transition between orbitals that appear as a result of the gold-lead interaction. Thus Fenske-Hall molecular orbital calculations indicated that the HOMO is constituted from the 6pz orbital of gold and 6s orbital of lead and the LUMO is entirely constituted from the 6pz orbitals of these atoms. 

In parallel to these lifetime changes, complexation induces tremendous modifications of the emission spectra, that have been examined for lanthanides in dedicated papers (for example, Hnatejko et al., 2000) or reviewed (Biinzli and Choppin, 1989). Briefly, for Eu(III), taken as a typical example of R(III) ions, the emission arises from the 5Do -> 7Fj transitions, from which the 5Do -> 7F2 (around 616 nm) exhibits hypersensitivity. Therefore, the wealth of information experimentally obtained is highly dependent on the scanning step used and a high resolution is needed to make the best of an emission spectrum (Biinzli and Choppin, 1989). However, even in the case of a low resolution, the changes are spectacular, as illustrated in fig. 7. 

Figure 8.29(b) shows that an L emission XRF spectrum is much more complex than a K emission spectrum. This is illustrated by the L spectrum of gold in Figure 8.31. Apart from those labelled t and r, the transitions fall into three groups, labelled a, (3 and y, the most intense within each group being a, (5, and y1 respectively.

In the UVA is absorption spectrum, a broad band at labs = 370 nm and a more intense one at 292 nm are attributed to transitions to the MLCT and the ( LC) excited states, respectively. Such characteristics are common for many rhe-nium(I) diimine carbonyl complexes (Table I). The emission spectrum comprises a single broad band, which arises from a transition from the lowest MLCT excited state to the groimd state. The lifetime of the MLCT excited state is relatively short (Table I). 

Souz et al. synthesized and structurally characterized a tetramer complex [Eu4(ETA)9(OH)3 (H20)3l (see Figure 2.42 [76], where ETA = ethyl 4,4,4-trifluoroacetoacetate. From these structural data, they calculated the ground-state geometry of the tetramer by using the Sparkle/AMl model. The emission spectrum shows that the Dq Fq transitions in the emission spectrum are consistent with the Eu + ion occupying four different sites in chemical environments of low symmetries. 

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