Triplet transition

These hold quite well for light atoms but become less dependable with greater nuclear charge. The tenu mtercombination bands is used for spectra where the spin quantum number S changes for example, singlet-triplet transitions. They are very weak in light atoms but quite easily observed in heavy ones. 

Fig. 5 Schematic representation of the electronic transitions during luminescence phenomena [5]. — A absorbed energy, F fluorescence emission, P phosphorescence, S ground state. S excited singlet state, T forbidden triplet transition.

Fig. 14 Schematic representation of the electronic transitions of photochemically excited substances Sq = ground state, Sj = first excited singlet state, T = forbidden triplet transition, N = ground state of a newly formed compound, A = absorption, F = fluorescence, P = phosphorescence.

Table 3.3. High Singlet and Triplet Transitions by Flash Photolysisa...

In other words, in this approximation for electronic states given by Eqs. (3.87) and (3.88) the triplet-triplet transition is due to the exchange interaction. 

Next we consider the triplet-triplet transition. In the Condon approximation we have... 

In connection with Chichibabin s hydrocarbon it is appropriate to mention that there is little or no resonance between states of different multiplicity.4 Thus the singlet-triplet transition represented by the covalent and diradical structures of the hydrocarbon is a true equilibrium and not an example of resonance. 

An early report [110] of the occurrence of a singlets triplet transition in an apparently six-coordinate complex has recently been shown to be a fur-... 

Spin selection rule An electronic transition takes place with no change in the total electron spin - that is, AS = 0 - hence singlet <- triplet transitions are forbidden or very weakly allowed. For example, the S0 —> Ti transition in anthracene has a molar absorption coefficient, emax, some 108 times less than that corresponding to the S0 —> Si transition. 

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... 

The Perrin-Jablonski diagram (Figure 3.1) is convenient for visualizing in a simple way the possible processes photon absorption, internal conversion, fluorescence, intersystem crossing, phosphorescence, delayed fluorescence and triplet-triplet transitions. The singlet electronic states are denoted S0 (fundamental electronic state), Si, S2,... and the triplet states, Ti,T2,. Vibrational levels are associated with each electronic state. It is important to note that absorption is very fast ( 10 15 s) with respect to all other processes (so that there is no concomitant... 

Once a molecule is excited and reaches triplet state Ti, it can absorb another photon at a different wavelength because triplet-triplet transitions are spin allowed. These transitions can be observed provided that the population of molecules in the triplet state is large enough, which can be achieved by illumination with an intense pulse of light. 

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