Fluorescence decay phenomenon

The excited state of a molecule can last for some time or there can be an immediate return to the ground state. One useful way to think of this phenomenon is as a time-dependent statistical one. Most people are familiar with the Gaussian distribution used in describing errors in measurement. There is no time dependence implied in that distribution. A time-dependent statistical argument is more related to If I wait long enough it will happen view of a process. Fluorescence decay is not the only chemically important, time-dependent process, of course. Other examples are chemical reactions and radioactive decay. 

Fig. 23. The fluorescence decay of Cd vapor in a magnetic field, (a) Experimental data exhibiting the phenomenon of quantum beats, (b) The exponentially decaying component, (c) The decaying modulated component. This figure is reproduced from the work of Dodd, Kaul, and Warrington (158).

Work conducted by Tiller and Jones (1997) demonstrated that the fluorescence of PAHs decayed over time under both under anoxic and oxic conditions. Typically, however, the presence of dissolved oxygen had a more pronounced influence on baseline fluorescence decay for all the PAHs studied. Moreover, certain PAHs (pyrene and anthracene) were more susceptible to this phenomenon than others. To date a mechanism to explain this phenomenon has not been identified, but it is probably a combination of complex pathways including the reaction of the analyte with reactive oxygen species formed from the excited triplet state DOM and the direct photolysis of the analyte by the excitation light source. Thus, the application of fluorescence quenching for measuring Kdom is probably limited to systems, which can be analyzed under anoxic conditions. 

It is from only this lowest first excited singlet state, that the decay phenomenon known as fluorescence may occur. This is denoted in Figure 16.2 as F. In order for these processes to occur, it takes very unique organic structures. 

The goal for this analysis was to measure the prompt fluorescence decay time for each polycrystalline YSZ thickness. Results from this analysis are shown in Table 2. Visible fluorescence was observed through all three YSZ thicknesses at each of the tested laser wavelengths. It was not possible to measure the prompt fluorescence decay time for YAG Eu because the reduction of light intensity did not follow a simple exponential curve. This phenomenon is most likely caused by electrical noise, stray light, or the absorption and re-emission of fluorescence from the YSZ. 

Finally, a phenomenon called concentration quenching or static quenching can lead to upward curvature of Stern Volmer plots even at moderate quencher concentrations (c q > 0.01 M). Molecules that are located next to a quencher at the time of excitation will be quenched immediately. Therefore, the fluorescence decay curve will be nonexponential initially, exhibiting a very fast initial component. Moreover, the initial depletion of these molecules will result in an inhomogeneous distribution of the remaining excited molecules around the quenchers. As a result, the diffusion coefficient kA is no longer a constant, but becomes a function of time, kd(t), until the statistical distribution of excited molecules is re-established. The impact of these effects has been analysed in detail.231 Intrinsic rates of electron transfer in donor acceptor contact pairs can be extracted from the resulting curvature in Stern Volmer plots.232... 

The situation changed, however, with two advances. The first advance was the discovery that in the S, - S0 spectrum of jet-cooled anthracene a second band exists (at S, + 1420 cm-1), the excitation of which gives rise to quantum beat-modulated fluorescence decays.40 Besides indicating a somewhat more global importance to the beat phenomenon in anthracene, the characteristics of these new beats provided very strong evidence that they arose as a manifestation of IVR. In particular, the beats were shown to have phases and modulation depths dependent on the fluorescence band detected. Such behavior, which... 

They confirmed that the dwell time of excitations is significantly shortened as the detection window is scanned towards the high-energy tail of the fluorescence spectrum. A more recent example of this phenomenon is the spectrally resolved fluorescence decay from a neat PhPPV film, employing the streak-camera technique [66]. 

Fluorescence and phosphorescence are both forms of luminescence [3]. If the emission of radiation has decayed within 10 s after the exciting radiation is cut off it is known as fluorescence [4], if the decay phase lasts longer (because the electrons return to the ground state from a forbidden triplet state (Fig. 5), then the phenomenon is known as phosphorescence. A distinction is also made between... 

G(t) decays with correlation time because the fluctuation is more and more uncorrelated as the temporal separation increases. The rate and shape of the temporal decay of G(t) depend on the transport and/or kinetic processes that are responsible for fluctuations in fluorescence intensity. Analysis of G(z) thus yields information on translational diffusion, flow, rotational mobility and chemical kinetics. When translational diffusion is the cause of the fluctuations, the phenomenon depends on the excitation volume, which in turn depends on the objective magnification. The larger the volume, the longer the diffusion time, i.e. the residence time of the fluorophore in the excitation volume. On the contrary, the fluctuations are not volume-dependent in the case of chemical processes or rotational diffusion (Figure 11.10). Chemical reactions can be studied only when the involved fluorescent species have different fluorescence quantum yields. 

Another less-common form of unimolecular decay is also temperature dependent and results in the phenomenon known as is-type delayed fluorescence.183 For example, the lowest excited singlet and triplet states of eosin are quite close together in energy, so that excited... 

Edmond Becquerel (1820-1891) was the nineteenth-century scientist who studied the phosphorescence phenomenon most intensely. Continuing Stokes s research, he determined the excitation and emission spectra of diverse phosphors, determined the influence of temperature and other parameters, and measured the time between excitation and emission of phosphorescence and the duration time of this same phenomenon. For this purpose he constructed in 1858 the first phosphoroscope, with which he was capable of measuring lifetimes as short as 10-4 s. It was known that lifetimes considerably varied from one compound to the other, and he demonstrated in this sense that the phosphorescence of Iceland spar stayed visible for some seconds after irradiation, while that of the potassium platinum cyanide ended after 3.10 4 s. In 1861 Becquerel established an exponential law for the decay of phosphorescence, and postulated two different types of decay kinetics, i.e., exponential and hyperbolic, attributing them to monomolecular or bimolecular decay mechanisms. Becquerel criticized the use of the term fluorescence, a term introduced by Stokes, instead of employing the term phosphorescence, already assigned for this use [17, 19, 20], His son, Henri Becquerel (1852-1908), is assigned a special position in history because of his accidental discovery of radioactivity in 1896, when studying the luminescence of some uranium salts [17]. 

In early studies on fast fluidization (Yerushalmi et al., 1979 Li and Kwauk, 1980a), clustering of particles was recognized as a fundamental phenomenon, and much attention has since been given to the visualization of bed structure. Qin and Liu (1982) developed a fluorescent particle visualization technique, as shown in Fig. 10. Particles covered with a long-decay fluorescent powder... 

In addition to the field enhancement, the increases of the radiative decay rate of the molecule also lead to the fluorescence enhancement. This happens when molecules are S -20nm away from metal nanoparticies aggregated on surfaces [19-21]. Lakowicz and coworkers have characterized this phenomena by using silver island films deposited on the internal surface of two quartz plates which sandwich a bulk fluorophore solution [20]. The fluorophores are physically placed close to silver islands so that there are a range of distances between the fluorophore and metal. The fluorescence enhancement is accompanied by decreased lifetimes and increased photostability. This phenomenon shows that the silver island increases the radiative decay rate of the fluorophore and therefore induces the fluorescence enhancement. 

Energy migration may reveal itself as a fast component in the decay of fluorescence of M, being often in the pfcosecond region, but possibly in some polymer systems on the nanosecond time scale. The phenomenon can certainly contribute greatly to the observed depolarization of fluorescence ... 

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