Spin allowed transitions

Singlet oxygen, O2, can readily be generated by irradiating normal triplet oxygen, 2 in the presence of a sensitizer, S, which is usually a fluorescein-type dye, a polycyclic hydrocarbon or other strong absorber of light. A spin-allowed transition then occurs ... 

Figure A Simplified Energy Level diagram for d ions showing possible spin-allowed transitions in complexes of low-spin cobalt(lll).

In a cubic field three spin-allowed transitions are expected because of the splitting of the free-ion, ground term and the presence of the term. In an octahedral field the splitting is the same as for the octahedral d ion and the same energy level diagram (p. 1029) can be used to interpret the spectra as was used for octahedral Cr Spectra of octahedral Ni usually do consist of three bands which are accordingly assigned as ... 

It is interesting that the very broad, so-called spin-allowed transitions, like most of those in Fig. 2-1, were not actually recognized as such until the 1950 s. This was because of the characteristics of the spectrograph rather than the spectrometer. 

In the next chapter we look at the intensities of d-d electronic transitions. We shall see that transitions between terms of the same spin-multiplicity are much more intense than those involving a change of spin. It is for this reason that our focus in the present chapter has been on the former. We have seen that for d d , d and configurations in octahedral or tetrahedral environments, there is only one so-called spin-allowed transition. For

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Regardless of the nature of the space parts, Q vanishes if V spin V spm- If Q vanishes, so does /. Thus we have the so-called spin-selection rule which denies the possibility of an electronic transition between states of different spin-multiplicity and we write AS = 0 for spin-allowed transitions. Expressed in different words, transitions between states of different spin are not allowed because light has no spin properties and cannot, therefore, change the spin. 

Consider now spin-allowed transitions. The parity and angular momentum selection rules forbid pure d d transitions. Once again the rule is absolute. It is our description of the wavefunctions that is at fault. Suppose we enquire about a d-d transition in a tetrahedral complex. It might be supposed that the parity rule is inoperative here, since the tetrahedron has no centre of inversion to which the d orbitals and the light operator can be symmetry classified. But, this is not at all true for two reasons, one being empirical (which is more of an observation than a reason) and one theoretical. The empirical reason is that if the parity rule were irrelevant, the intensities of d-d bands in tetrahedral molecules could be fully allowed and as strong as those we observe in dyes, for example. In fact, the d-d bands in tetrahedral species are perhaps two or three orders of magnitude weaker than many fully allowed transitions. 

Occasionally, some bands which might otherwise be expected to be weak are observed to be quite strong. Two examples are shown in Fig. 4-4. The first shows the electronic spectrum of a solution containing [CoC ] ions in nitromethane. For this cT system, we expect three spin-allowed transitions and these are observed at roughly 3500, 7000 and 14,000 cm h They correspond (see Chapter 3) to the excitations M2 —> Ti F) and T P) respectively. Note, however, that the... 

Sometimes, spin-allowed bands are much weaker than otherwise expected. There can be many reasons for this, most of which require more detailed analysis than we are able to present here. One particular case, however, can be discussed. It is well illustrated by the spectra of octahedral cobalt(ii) species, an example being shown in Fig. 4-5. Three spin-allowed transitions are expected for these d complexes, namely Txg F)- T2g, - see Chapter 3. The bands in Fig. 4-5 are... 

Again, we restrict discussion to spin-allowed transitions here. In general, of course, crystal field effects compete with interelectron repulsion for all d" configurations, exceptfor n = 1 or 9. 

The middle band of the three spin-allowed transitions of octahedral Ni11 complexes ( 2g 2g T g F), A 2g —> Tig P) in Oh symmetry) is often found to be split into two... 

Once again two spectroscopic studies have been made — due to Brown et al. (32) and to Allen et al. (12) respectively — and the same comments apply as for the two investigations of the RuFspin-forbidden bands at 12.2 and 16.1 kK., with spin-allowed transitions at 19—21 kK. and at 26.0 kK., but no other absorptions were found below 40 kK. On the other hand the spectrum of Allen et al., although broadly mirroring these findings, revealed extra absorptions at 32.8,39.6, and 44.6 kK., together with a strong indication of a band below 4 kK. (Fig. 2). 

The spin-allowed transitions for ions having T and A ground states in tetrahedral fields are as follows ... 

It can be seen from Table 18.5 that all excited spectroscopic states having a multiplicity that is different from the ground state have energies that are expressed in terms of both B and C. As we have seen from the previous discussion, spin-allowed transitions occur only between states having the same multiplicity. Therefore, in the analysis of spectra of complexes only B must be determined. It is found for some complexes that C 4B, and this approximation is adequate for many uses. 

FIGURE 18.5 Simplified Tanabe-Sugano diagrams ford" metal ions in octahedral fields. The drawings have been simplified by omitting several states that have multiplicities that do not permit spin allowed transitions. 

Thus the six spin-allowed transitions from the ground state 4Blg have energies which are given by the roots of a 3 x 3 determinant (E ), a 2 x 2 determinant (A2g) and a simple linear equation for B2g- In practice, only four transitions are observable ... 

The reducing character of the ligand is linearly related to the relative position of the first spin-allowed transition and the first electron transfer transition. Jorgensen 233) calculated the optical electronegativity of ethyl-dsep as 2.6 which is close to that of ethyl-dtp, 2.7. Jorgensen 233) has also discussed electron delocalization in M(X2P)3 (X = S, Se) chromophores in terms of a molecular orbital model. 

The most important free ion states are ground level and excited level. In an octahedral field, the level splits into the A2 groimd state and the excited T2 and states. The spin allowed transitions that could therefore be used to populate the excited states directly correspond to A2 T2 and... 

Chromium activated ruby was the first laser material and its luminescence properties are carefully studied. It is a classical example of Cr + in octahedral crystal field. Here Cr + substitutes the AP ions, while such a possibihty can be rationalized by an excellent chemical fit of Cr in place of Al. Ruby is a high crystal field material and thus the T2g state Hes above the E2g level. Pumping is accomplished by a spin-allowed transition into the state, while emission occurs from the level without vibrational broadening and almost all excited... 

AP+ (0.675 A). Thus from ciystallochemical positions the presence of Mn " in topaz structure is quite possible. Strong absorption bands due to Mn " corresponding to the spin-allowed transitions to levels in the visible to near-UV region generate the yellow color of the phosphors, which corresponds to the yellow color of topaz. Thus we propose that a connection may be considered between the long-lived lines at 697 and 711 nm and Mn + centers, which may be formed as a result of natural irradiation of topaz. 

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