Electronic spectra of f electron systems

For r 3 n 11, only a selection of terms is given, listed in approximate energy sequence (lowest first). A limited number is ven because mixing of the type shown in Fig. 8.16 occurs and particularly complicates the more highly excited states of those configurations for v rhich the total number of functions is large. 

As a class, the complexes of the lanthanides are both beautifully and delicately coloured. There is a symmetry in the colours of the ions in aqueous solution, a symmetry which seems to be accidental. These colours are given in Table 11.9, listed in a way which shows the colour parallel between f and f ions. Also shown are the colours of some organo-lanthanide compounds, which do not show anything like the same pattern, colours which support the idea that the colour pattern of the aqua ions is accidental. 

The actual absorption bands that occur in the spectra of the lanthanides and actinides and which are associated with electronic transitions have been divided into three types. 

f - f transitions These transitions are localized entirely within the f shell and so, like d d transitions, are formally forbidden. However, like d d, they actually occur and give rise to a large number of weak, sharp bands from the infrared through to the visible region. The absorption band patterns obtained from species in solution are closely related to the emission spectrum of the corresponding ion, obtained from the arc spectra of the elements. 

nf - (n + l)d transitions Here, the n and (n + 1) are principal quantum numbers so these are allowed bands in which a 4f (lanthanides) or 5f (actinides) electron is promoted to a 5d and 6d orbital, respectively. They give rise to quite intense and broad bands, lowered by ca. 15 000 cm 

---