Lifetime radiative, definition

Delayed Fluorescence. By definition the fluorescence emissions are spin-allowed radiative transitions of atoms or molecules they have short lifetimes, of the order of ns to a few hundred ns. There are however some cases where molecules emit the very same fluorescence spectra but with much longer decays and often with complex non-exponential kinetics. These... 

Confusion reigns when one examines the definitions of fluorescence and phosphorescence in different areas of the literature of the natural sciences. Many physicists in particular prefer the operational definition in which fluorescence is described as short-lived emission and phosphorescence is long-lived emission (4,15). However, the question arises as to what constitutes short-lived. A possible transition point may be radiative lifetimes, r0 (vide infra), of the order of 10-s to 10 6 sec. 

The mathematical definition of fluorescence lifetime arises from the fact that nonradiative and radiative processes participate in the fluorophore deexcitation. 

The first electronic transition in butadiene has been the subject of many experimental and theoretical studies.The absorption, which has a maximum at 2100 A., is strong and represents a tt tt transition from a ground singlet to an upper singlet state. Analysis of the spectrum, which shows very little structure, has not been carried out. Since no fluorescent radiation has ever been detected on excitation of any of the simple dienes even at low temperature, a definite assignment of the 0 — 0 band has not been made. The 0 — 0 band had been placed at 2300 A. (124 kcal./mole), at which point the absorption is only /so as intense as at its maximum. The oscillator strength is 0.53, which leads to a radiative lifetime of 10 sec. Since emission of radiation has not... 

The relationships between measurable quantities related to absolute transition probability (e.g. absorption cross section, molar absorption or extinction coefficient, radiative lifetime) and the fundamental quantities used to describe and inter-relate the observable quantities axe fraught with difficulties of unit conversions and internally consistent treatments of initial- and final-state degeneracies. Several excellent papers on this subject exist (Hilborn, 1982 and 2002, Larsson, 1983, Tatum, 1967, Schadee, 1978, and Whiting, et al., 1980). Much of Section 6.1.1 is based on or checked against Hilborn (1982 and 2002), although slightly different notation and definitions are used. 

In order to settle a controversy over the effects of deuteriation on the radiative lifetimes of aromatic hydrocarbons, some systems have been examined, using two independent techniques. Table 7 shows the rr values obtained from phosphorescence lifetimes and quantum yields. Evidence for the intramolecular nature of the deuterium isotope effect on rr comes from the lack of sensitivity to solvent. Although no definitive evidence as to the origin of the isotope effect is put forward, it is suggested that there may be two important contributing mechanisms to the 7i -> S0 radiational process, an isotope-independent and an isotope-sensitive one. The isotope-insensitive mechanism is almost certainly first-order spin-orbit coupling. It therefore remains to determine the nature of the other process, which will only be important when spin-orbit coupling is inefficient.1886... 

Inspection of the results in Table 5 suggests that the lifetime for the of Sq4 is 2.2 ns and is independent of solvent. Although the total for Sq4 in these solvents are high, they are definitely less than unity. As the radiative rate is not expected to be solvent-sensitive, the constant fluorescence lifetime... 

The measurement of the absolute quantum yield has to be completed by the determination of the hfetimes tobs and the radiative lifetimes trad, giving access to the intrinsic d>Ln quantum yields, and overall sensitization efficiency jjsens-These values are sometimes neglected however, they allow a better understanding of the electronic structure of the ligand and therefore of the influence of the ligand for sensitizing the lanthanide ion. They can allow a better prediction of the properties of the complexes. For a better definition of each of these values, the reader is referred to Luminescence, and for a discussion of the influence of these parameters within the heUcate famihes, to the article of Biinzli et al ... 

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