Spin-forbidden transitions intensities

The first two terms in the expansion are strictly zero because of the spin selection rule, while the last two are non-zero, at least so far as the spin-selection rule is concerned. So a spin-forbidden transition like this, X VT , can be observed because the descriptions X and are only approximate that is why we enclose them in quotation marks. To emphasize the spin-orbit coupling coefficients for the first row transition elements are small, the mixing coefficients a and b are small, and hence the intensities of these spin-forbidden transitions are very weak. 

Here we comment on the shape of certain spin-forbidden bands. Though not strictly part of the intensity story being discussed in this chapter, an understanding of so-called spin-flip transitions depends upon a perusal of correlation diagrams as did our discussion of two-electron jumps. A typical example of a spin-flip transition is shown inFig. 4-7. Unless totally obscured by a spin-allowed band, the spectra of octahedral nickel (ii) complexes display a relatively sharp spike around 13,000 cmThe spike corresponds to a spin-forbidden transition and, on comparing band areas, is not of unusual intensity for such a transition. It is so noticeable because it is so narrow - say 100 cm wide. It is broad compared with the 1-2 cm of free-ion line spectra but very narrow compared with the 2000-3000 cm of spin-allowed crystal-field bands. 

Figure 14 shows that the intensity of the Sb(III)-Sb V) MMCT transition is relatively weak. This follows from comparison with the Sb(III) transition in the ultraviolet which is spin forbidden. The intensity of the MMCT transition is determined by the amount of Sb(IV)Sb(IV) admixture in the Sb(III)Sb(V) ground state. From the intensity of the MMCT transition the amount of admixture into the ground state is found to be less than 1%. 

Spin selection rule The spin selection rule, AS = 0, specifies that there should be no change in the spin multiplicity. Weak spin-forbidden bands may occur when spin-orbit coupling is possible. Spin-forbidden transitions are more intense in complexes of heavy atoms as these lead to a larger spin-orbit coupling. 

The smaller intensity of the second band can probably be attributed to its forbidden character in limiting Dgn symmetry. In this spectrum there also are present other bands which can be attributed to spin-forbidden transitions and/or traces of cromium(III) (29). 

The bands at 14 and 16 kK, seem to be typical spin-forbidden transitions because of their weak intensity. 

CsNiCl3-type). Table 6 summarizes results on electronic spectra. The three spin-allowed transitions are usually observed and extensive vibrational contribution has been observed in single crystal spectra.120,121 Intense spin-forbidden transitions may also be observed122 due to exchange interactions in the solid phase. The diiodide123,124 and the complex halides121 show trigonal distortion. 

MCD spectroscopy has been useful in the location of the spin-forbidden transitions.1019 That the Cr—S bond is comparatively strong has been adduced from the presence of relatively intense molecular ion peaks in the mass spectra of several Cr111 dialkyldithiocarbamates.1020... 

The spin selection rule breaks down somewhat in complexes that exhibit spin-orbit coupling. This behavior is particularly common for complexes of the heavier transition elements with the result that bands associated with formally spin forbidden transitions (generally limited to AS — s ) gain enough intensity to be observed. Table 11.16 summarizes band intensities for various types of electronic transitions, including fully allowed charge transfer absorptions, which will be discussed later in the chapter. 

The octahedral complexes of these ions are, of course, all of the spin-paired type.38 The values of Dq are higher than for, say Co111, so that usually only the Tlg band is observed below the charge transfer bands. The spin-forbidden transitions to the 3Tlg and 3T2g terms can be quite intense (e a 10), presumably because of the very large spin-orbit coupling in some of these ions. Square-planar complexes have been studied.149... 

The decay time of this emission is very long, viz. some 5 ms [57,58]. There are two reasons for this. First the transition involved is spin forbidden [48, 51] secondly, the spin-allowed transition from which the spin-forbidden transition steals its intensity is unusually weak [58, 59],... 

As elaborated in detail in Ref. (5) there are two principal intensity mechanisms for dimer excitations. The single-ion mechanism is based on the combined action of spin-orbit coupling and an odd-parity ligand field potential at the Cr center. It is by this mechanism that spin-forbidden transitions obtain their intensity in mononuclear complexes. The pair mechanism, on the other hand, is restricted to exchange-coupled systems. It leads to the selection rules AS = 0,... 

Taking into account all of the factors influencing intensities of crystal field spectra discussed so far, the following generalizations may be made. Transitions of 3d electrons within cations in octahedral coordination are expected to result in relatively weak absorption bands. Intensification occurs if the cation is not centrally located in its coordination site. In tetrahedral coordination, the intensities of crystal field transitions should be at least one-hundred times larger than those in octahedrally coordinated cations. Spin-forbidden transitions are usually about one-hundred times weaker than spin-allowed transitions in centrosymmetric, octahedrally coordinated cations, but become... 

The hue or vividness of colour may be correlated with the intensities of the absorption bands. Thus, Al-Fe epidotes, with relatively low molar extinction coefficients typical of spin-forbidden transitions within Fe3+ ions ( 3.7.2), exhibit pastel shades. The Al-Mn-Fe epidotes, however, display vivid colours correlating with high e values and originating from spin-allowed transitions within Mn3+ ions located in the very distorted acentric octahedral M3 site (fig. 

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