Spin-forbidden absorption

The ds correlation diagram (Fig. Il.37d) is particularly interesting. The ground state CA iP is the only state on the diagram with a multiplicity of 6. This means that for a d5 octahedral complex, all transitions are not only La pone forbidden but also spin forbidden. Absorptions associated with doubly forbidden transitions are extremely weak, with extinction coefficients several hundred times smaller than those for singly forbidden transitions. It is understandable, then, that dilute solutions of Mn(ll) are colorless and only with a substantial increase in concentration is the characteristic faint pink color of [Mn(H20)6]24 observable (Fig. 11.38b). 

Fig. 31. Temperature dependence of the intensity of the formally spin-forbidden absorption band A] Aj + E for Mn(II) in [Mn(Me4-[14]ane-N4)Cu(oxpn)](CF3S03)2 (A) experimental data, (—) calculated curve. Reprinted from Mathoniere et al. (56). Copyright 1993 American Chemical Society.

As stated in Chapter 1, transitions involving a change in multiplicity are spin forbidden. However, for reasons which we will consider later, such transitions do indeed occur although with very low transition probabilities in most cases. The intensity of an absorption corresponding to a transition from the ground state S0 to the lowest triplet state Tx is related to the triplet radiative lifetime t ° by the following equation[Pg.114]

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). 

Sometimes the atoms (or molecules) in molecular beams are put into selected electronic, vibrational and rotational states. The initial state selection can be made with lasers. A laser beam of appropriate frequency is shined onto a molecular beam and the molecule goes onto an appropriate excited state. The efficiency of selection depends upon the absorption coefficient. We can attain sufficient absorption to get highly vibrationally excited molecule with the laser. A spin forbidden transition can also be achieved by using a laser. 

Population of the triplet manifold by direct singlet-triplet absorption is a very inefficient process, being spin-forbidden. Instead, the triplet manifold is populated indirectly by excitation into the singlet manifold followed by intersystem crossing. 

Since the photon emitted by D is absorbed by A, the same rules will apply to radiative energy transfer as to the intensity of absorption. Because singlet-triplet transitions are spin-forbidden and singlet-triplet absorption coefficients are usually extremely small, it is not possible to build up a triplet state population by radiative energy transfer. For this... 

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