Triplet absorption coefficients

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... 

Fig. 22 B. Exeitation spectrum at room temperature showing the intensity of delayed fluorescence of a naphthalene crystal as a function of the wavelength of the exciting light. The ordinate is proportional to the square of the singlet-triplet absorption coefficient. (From Avakian and Abramson, Ref.52))...

The FDS5 pyrolysis model is used here to qualitatively illustrate the complexity associated with material property estimation. Each condensed-phase species (i.e., virgin wood, char, ash, etc.) must be characterized in terms of its bulk density, thermal properties (thermal conductivity and specific heat capacity, both of which are usually temperature-dependent), emissivity, and in-depth radiation absorption coefficient. Similarly, each condensed-phase reaction must be quantified through specification of its kinetic triplet (preexponential factor, activation energy, reaction order), heat of reaction, and the reactant/product species. For a simple charring material with temperature-invariant thermal properties that degrades by a single-step first order reaction, this amounts to -11 parameters that must be specified (two kinetic parameters, one heat of reaction, two thermal conductivities, two specific heat capacities, two emissivities, and two in-depth radiation absorption coefficients). 

Excited states of N20, undergoing reactions (50), (51), and (52), are all triplets, transitions to which are spin forbidden. A large absorption coefficient (e = 200) at this wavelength appears to eliminate these processes, but it is possible that these triplet states may interact with neighboring singlet states which makes an intercombination possible.74... 

Because of their special polymerization process [136], PDA crystals should not contain any dopant or associated residual polaron or bipolaron concentration. These negative results give some constraints on other low-lying states, especially exciton states. Triplet-state absorption from the ground state would probably be too strongly spin-forbidden to have absorption coefficients as high as 1 cm-1 [129]. As for g states, the absorption coefficient depends sensitively on how far from the intense absorption they are (for the polyene case, see Ref. 127). That they are not found in these experiments means that they are either above the main transition at 2 eV or not far below it and buried in its tail. Indeed, evidence of a weak absorption = 0.1 eV below the main transition has recently been found at low temperature [118] it would be buried in the absorption tail at higher temperatures. 

From these activities, the triplet-triplet absorption spectrum of fenbufen in acetonitrile was confirmed and the molar absorption coefficient of the absorption at 420 nm was determined, as discussed in the next section. Xanthone was used as the sensitizer to confirm the triplet-triplet absorption spectrum of benoxaprofen (Navar-atnam et al., 1984, 1985) and mefloquinone in water (Navaratnam et al., 2000) benzophenone as the sensitizer for benoxaprofen and derivatives in acetonotrile (Navaratnam et al., 1993) and acetonaphthone as a quencher for ofloxacine in water (Navaratnam and Claridge, 2000). Selection of a sensitizer depends on a number of factors for example, the sensitizer must be such that... 

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