Fluorescence lifetimes, definition

Indeed, the laser oscillation itself frequently is a transient emission that falls into the broad definition of fluorescent lifetimes. In particular, spiking phenomena (5) and Q Switching (4) clearly bear a close relationship to relaxation times and processes. One could say that laser phenomena are associated with the study of fluorescent decays of inverted populations. 

B) Phase-shift methods. The phase shift method for determining fluorescence lifetimes is based on the principle that if fluorescence is excited by suitably modulated light source, emitted radiation will also be similarly modulated. With reference to a scattering substance, emission from a fluorescent substance will introduce a time lag due to finite time between absorption and emission. This, by definition is the lifetime of the excited state. The time lag will cause a phase-shift relative to the exciting light. Phase fluorimetry requires a modulated light source and a phase sensitive detector. 

The lifetime of fluorescence (or phosphorescence), r0, is defined using the rate constant kf as r0 = 1 /k. By this definition, the residual intensity according to equation (12.1) is only 36.7% of the initial intensity at r0 63.3% of the initial species have relaxed to a non-emissive state. Because fluorescence lifetimes are only a few nanoseconds, fluorimeters require measurements to be made at the same time as... 

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... 

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

Since the gas phase data of ER, which does not have sufficient vapour pressure at accessible temperatures, is not available, it is difficult to say definitely, but from the pictorial representation of APS-ER given in Fig.1 the ER moiety of APS-ER may well be expected to be partially in the gas phase. Consequently, a high collision frequency of oxygen may be responsible for the shortened fluorescence lifetime of APS-ER in the air. 

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... 

It is clear from the examples we have given and from the definitions of the two types of quenching, that static quenching occurs when a complex is formed between two molecules. Collisional quenching does not need the formation of a stable complex with a long lifetime. The fluorophore and the quencher enter into collision inducing the decrease of the fluorescence lifetime and intensity of the fluorophore. 

Thus, taking into consideration the above definitions, it will be possible to distinguish between the two types of quenching without measuring the fluorescence lifetime at different quencher concentrations. 

This expression is similar to that which will be deduced in the next chapter for the yields of products of parallel first-order (or pseudo-first-order) reactions, such as reactions (3.XIV)-(3.XVI). From the definition of experimental molecular fluorescence lifetimes Tq = l/( f + the previous equation reduces to... 

Fig. 1.6. Strategy for the choice of a fluorescent probe. Av, , and t are the Stokes shift, quantum yield and lifetime, respectively (see definitions in Chapter 3).

Triplet—triplet energy transfer from benzophenone to phenanthrene in polymethylmethacrylate at 77 and 298 K was studied by steady-state phosphorescence depolarisation techniques [182], They were unable to see any clear evidence for the orientational dependence of the transfer probability [eqn. (92)]. This may be due to the relative magnitude of the phosphorescence lifetime of benzophenone ( 5 ms) and the much shorter rotational relaxation time of benzophenone implied by the observation by Rice and Kenney-Wallace [250] that coumarin-2 and pyrene have rotational times of < 1 ns, and rhodamine 6G of 5.7 ns in polymethyl methacrylate at room temperature. Indeed, the latter system of rhodamine 6G in polymethyl methacrylate could provide an interesting donor (to rose bengal or some such acceptor) where the rotational time is comparable with the fluorescence time and hence to the dipole—dipole energy transfer time. In this case, the definition of R0 in eqn. (77) is incorrect, since k cannot now be averaged over all orientations. 

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 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... 

---