Lifetime fluorescence decay time

Fluorescence Lifetimes. The fluorescence decay times of TIN in a number of solvents (11.14.16.18.19), low-temperature glasses (12.) and in the crystalline form (15.) have been measured previously. Values of the fluorescence lifetime, Tf, of the initially excited form of TIN and TINS in the various solvents investigated in this work are listed in Table III. Values of the radiative and non-radiative rate constants, kf and knr respectively, are also given in this table. A single exponential decay was observed for the room-temperature fluorescence emission of each of the derivatives examined. This indicates that only one excited-state species is responsible for the fluorescence in these systems. 

From a practical point of view the consequences of TOF dispersion are important only for short intrinsic fluorescence decay times of to < 1 nsec. Figure 8.15 shows an example with to = 50 psec and realistic optical constants of the substrate. The intensity maximum in Fb(t) is formed at At 30 psec after (5-excitation. After this maximum, the fluorescence decays with an effective lifetime of r ff = 100 psec that increases after long times to t > > 500 psec. The long-lived tail disappears as soon as there is some fluorescence reabsorption, and for Ke = K there is practically no difference to the intrinsic decay curve (curve 3 in Figure 8.15). 

There are many molecular interactions which influence the fluorescence decay times. The measured fluorescence lifetime r is usually shorter than the radiative lifetime tr because of presence of other decay rates which can be dependent on intramolecular processes and intermolecular interactions (Figure 10.3). The measured fluorescence lifetime (r) is given by the inverse of the total rate of dynamic processes that cause deactivation from the excited (mostly singlet Si) state... 

The spontaneous fluorescent decay time tf is connected with the radiative lifetime tfr and the quantum yield of fluorescence ijf by jjf =tf/tfr. Since the radiative lifetime is of the order of a few nanoseconds in most dyes, the spontaneous fluorescent decay time is about the same for quantum yields of fluorescence near unity (i.e., k Q ttksT 0) and decreases to a few picoseconds for quantum yields of the order of 10-3. 

Geusic et al. (118) made measurements of the fluorescent lifetime of neodymium in yttrium aluminum garnet (Y3A15012). For neodymium concentrations up to 3 atomic per cent, the measured fluorescent-decay time is approximately 200 jtxsec at both IT and 300°K. Above 6 atomic per cent, a marked decrease in the fluorescent lifetime is observed. They suggest that this is due to neodymium interactions. It is to be noted that yttrium aluminum garnet is a laser material of very exceptional quality. 

It is clear that, by changing the experimental conditions and/or detection wavelength, limiting values can be found for all of the quantities mentioned above from measurements of the fluorescence decay time. The effects of collisional and spontaneous processes can be separated by conventional Stem—Volmer analysis [36]. The concentration, [M], of quenching molecules is varied and the reciprocal of the observed lifetime is plotted against the concentration of M. The quenching rate coefficient is thus obtained from the slope and the intercept gives the rate coefficient for the spontaneous relaxation processes, which is usually the natural lifetime of the excited state. In cases where the experiment cannot be carried out under collision-free conditions, this is the only way to measure the natural lifetime from observation of the fluorescence decay. 

We can provide the following summary for the decay behavior of simple aliphatic aldehydes and ketones with little or no vibrational excitation energy on the Sp manifold under "isolated" molecule conditions at room temperature. A typical fluorescence decay time (tp) measured by a single-photon time-correlated lifetime apparatus (248) is 2-5 ns (42,101,102). A typical fluorescence quantum yield (ketones measured by fluorescence excitation spectroscopy is 10-, but the value is somewhat lower for aliphatic aldehydes (101,102). These values indicate that the radiative process (kp) is lO -lO s-1, three orders of magnitude slower than the total rate of nonradiative processes (kpjp) of 10 10 s-1. A typical radiative lifetime (tr) is 0.1 0.5 ps for aliphatic aldehydes and 0.1 ps for aliphatic ketones. 

Fluorescence lifetime measurements are an important aspect of photophysical research. In the past few months the phase-shift measurement technique has become more widely used. This is largely due to the successful achievement of a multifrequency modulation apparatus. An apparatus made from commercially available components has been described and shown to have an accuracy of 10 ps. The performance was checked using mixtures of acridine and quinine sulphate and least-squares-ht procedures. A series of papers from the Illinois group give very detailed account of the state of the art and show the power of the method. The colour delay error arising from the wavelength error in photodetectors can be determined and fluorescence decay times can be obtained with an accuracy of a few picoseconds. ... 

Ono and Ware"" have measured the absorption, emission, and excitation spectra, the fluorescence decay times, and the quantum yields of a series of substituted diphenylmethylenes in rigid matrices at low temperatures. Acean-thrylene shows S2- So emission in hexane with a yield of 0.017 and lifetime of 4.3 ns. The low-temperature fluorescence spectra of bis-2-naphthyl-alkanes and their derivatives have been studied. Excimer formation is an activated process. The fluorescence and absorption spectra of 1,1-diphenyl-ethylenes have been analysed in some detail by Gustav and Bolke. " The S — Si transitions in trans isomers of phenylnaphthylethylenes have been assigned by picosecond absorption spectroscopy. Effects of solvent viscosity and the role of conformers in the mechanism of isomerization are elucidated. The production of non-equilibrium conformer concentrations in glassy solutions of diarylethylenes at 77 K due to restrictions imposed by the solid matrix has also been reported. Free jet excitation and emission spectra of diphenyl-butadiene show clearly the lowest excited Ag state and give a lifetime of 52.8 ns for 0-0 excitation.Electric field-induced charges in the optical... 

The emission lifetime is often used to determine surface temperature with the advantage that the technique is insensitive to blackbody background. This technique requires excitation by a pulsed source, the persistence of the resulting fluorescence can be observed providing that the length of the source pulse is much shorter than the persistence time of the phosphor s fluorescence. For certain phosphors, the prompt fluorescence decay time (t) varies as a function of temperature and is deflned by ... 

As mentioned above radiationless energy transfer affects both fluorescence yield and fluorescence decay time. The influence of acceptor concentration on the donor lifetime is shown in Figs. 6 and 7. Here the lifetimes of the donor/acceptor combinations relative to the lifetime of the undisturbed donor are plotted as a function of the reduced acceptor concentration,y. ... 

A fluorescence decay time is a measurement, at fixed wavelength, of fluorescence signal as a function of time. Despite the fact that this time is very short, certain instruments can measure the lifetime of fluorescence. There exist several methods based, either upon the recording of the decrease curve of the light intensity... 

Table 11. Fluorescence Lifetimes and Decay Times of the Charge-Separated States in Dyads 5-8 in Benzonitrile Solution...

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