Selection rule, electric dipole hypersensitive transition

The intensities of the majority of the f-f transitions vary only within a factor of 2-3 from host matrix to host matrix, but some transitions are much more host dependent. These transitions are called hypersensitive transitions. These induced electric dipole transitions obey the selection rules for electric quadrupole transitions and are therefore sometimes called pseudo-quadrupole transitions. In sect. 8, we will discuss hypersensitivity in detail. The dependence of the Qx intensity parameters on the host matrix is the subject of sect. 9. Two-photon spectra (sect. 10) and vibronic transitions (sect. 11) are discussed briefly. On the other hand, chiroptical methods will not be considered. Since the color of the lanthanide ions is related to the spectral intensities of f-f transitions, we want to give attention to the phenomenon of color (sect. 12). Finally, the intensities of actinide ions are reviewed (sect. 13). 

The electric quadrupole transition arises from a displacement of charge that has a quadrupolar nature. An electric quadrupole consists of four point charges with overall zero charge and zero dipole moment. It may be pictured as two dipoles arranged so that their dipole moments cancel. An electric quadrupole has even parity. Electric quadrupole transitions are much weaker than magnetic dipole and induced electric dipole transitions. At this moment no experimental evidence exists for the occurrence of quadrupole transitions in lanthanide spectra, although some authors have claimed the existence of such transitions (e.g. Chrysochoos and Evers 1973). However, the so-called hypersensitive transitions (see sect. 8) are eonsidered as pseudo-quadrupole transitions, because these transitions obey the selection rules of quadrupole transitions. 

The intensities of the induced electric dipole transitions in lanthanide ions are not much affected by the environment. The dipole strength of a particular transition of a lanthanide ion in different matrices will not vary more than a factor two or three. However, a few transitions are very sensitive to the environment, and these are usually more intense for a complexed lanthanide ion than for the lanthanide ion in aqueous solution. The intensity increases up to a factor 200 (Gruen and DeKock 1966, Gruen et al. 1967). Only in a few cases has a lower intensity than in the aqueous solution been reported for these transitions (e.g. Krupke 1966). Jorgensen and Judd (1964) have called such transitions hypersensitive transitions. They noted that all known hypersensitive transitions obey the selection rules A5 = 0, AI 2 and jAJj 2. These selection rules are the same as the selection rules of a pure quadrupole transition, but calculations have revealed that the intensities of hypersensitive transitions are several orders of magnitude too large for these transitions to have a quadrupole character. Therefore, hypersensitive transitions have been called also pseudo-quadrupole transitions. No quadrupole transitions have been observed for lanthanide ions, although Chrysochoos and Evers (1973) stated that the intensity of the hypersensitive transitions D2 Fq (in the absorption spectrum) and Do Fi (in the luminescence spectrum) of Eu " are mainly quadrupolar in nature. 

Hypersensitive transitions are electric dipole transitions whose shape and intensity display large dependence on the point group symmetry of the metal ion, as well as on the pH, temperature, and ligand type. These transitions obey the following selection rules. 

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