F -> d transitions

Prediction of the energy level structure for Pu2+ (5f ) is of particular interest since no spectra for this valence state of Pu have been reported. On the basis of what is known of the spectra of Am2+ (26), Cf2" (27), and Es2+ (28), there appears to be evidence for a very small crystal-field splitting of the free-ion levels. Such evidence encourages use of a free-ion calculation in this particular case. The parameter values selected are indicated in Table V. Based on the systematics given by Brewer (19), the first f- d transition should occur near 11000 cm-, so the f- -f transitions at higher energies would be expected to be at least partially obscured. A... 

The f d transition in divalent rare earth ions occurs quite early and this broad intense / - -d band almost invariably blots out the weak f—f lines. 

The Ln2+ ions are often highly colored. This arises because the 4f orbitals in Ln2+ are destabilized with respect to those in Ln3+, and hence lie closer in energy to the 5d orbitals. This change in orbital energy separation causes the f->-d transitions to shift from the ultraviolet into the visible region of the spectrum. 

Solution absorption spectra of Bk(III) and Bk(IV) are shown in Figs. 2 and 3, respectively. The spectrum of Bk(III) is characterized by sharp absorption bands of low molar absorptivity attributed to Laporte-for-bidden f-f transitions and by intense absorption bands in the ultraviolet region, which are attributed to f-d transitions (96). The spectrum of Bk(IV) is dominated by a strong absorption band at 250-290 nm, the peak position of which is strongly dependent on the degree of complex-ation of Bk(IV) by the solvent medium. This band is attributed to a charge-transfer mechanism (96). 

In the case of Sm2+ (4f6), Eu2+ (4f7) and Yb2+ (4f14) electronic absorption spectra have been observed because they have lower-lying 4f" 15d configuration than the trivalent lanthanides with f configuration. In all the three cases, only f-d transitions are observed in solution. The weak f-f transitions of Sm2+ and Eu2+ are masked by the intense f-d bands. 

The onset of a 4/ — 5d transition for the trivalent rare earth ions usually takes place in the vacuum ultraviolet. In the case of Ce3+, Eu3+ and Tb3+ aquo ions the f - d transition occurs within the measurable range of most modern spectrophotometers (Fig. 20). 

The study of f-d transitions of Ln3+ in M2ALnX6 systems is in its infancy, but promises to yield more clearly-resolved spectra than for the lower-symmetry hosts studied thus far. This is partly because of the higher degeneracy of CF levels in the elpasolite hosts, leading to fewer possible transitions, as well as to the more restrictive selection rules pertaining to these transitions. Resonant energy transfer in elpasolite systems is well-understood, but the understanding and calculation of nonresonant processes is far from satisfactory. 

Both f-f and f-d transitions have been used for lanthanide and actinide lasers. The spectroscopic properties of these transitions are compared in Table I. Since the d states have shorter lifetimes, faster pumping as well as higher energies are required for excitation. Possible pumping sources include ultrafast flashlamps, other lasers, electron beams, or synchrotron radiation, with one exception, all lanthanide and actinide lasers have been optically pumped. 

There are several absorption bands in the visible and near-infrared (120) which could be used for optical pumping in hosts with strong ion-phonon coupling and efficient decay to the metastable J=ll/2 state. In some hosts, efficient emission and possible laser transitions may also occur from the higher J=ll/2 state at =11,500 cm- and the J=5/2 state at 20,000 cnH. Intense absorption bands begin at 33,000 cm-l and have been attributed to charge transfer states rather than f-d transitions (99). 

Although the transitions from the 4f levels were predicted to be in this frequency range, they were not seen in these experiments. The authors discussed the absence of f level transition in terms of the Fano and Cooper theory (1968). The f-d transition is allowed but is ten times weaker than the f-g... 

Dorenbos P (2000) The f-d transitions of the trivalent lanthanides in halogenides and chalco-genides. J Lumin 91 91-106... 

Relationship of f-d Transition with Structure and Composition in Nitrides... 

As described in the previous three sections, substitution of Si with Al in nitride phosphors generally leads to a red shift in emission spectra, whereas replacement of N by O leads to a blue shift. This can be understood by the fact that the f-d transition is related to the structure and composition of the host materials. 

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