Decay curve lifetime measurements

This work investigated PIM-1 membranes in the three states discussed above. Nickel-foil supported NaCl was used as a positron source and stacks of film samples, each about 1mm thick, were placed either side of the source. Annihilation lifetime decay curves were measured with an EG G Ortec fast-fast lifetime spectrometer. Measurements were made both in air and under an inert atmosphere (N2). However, o-Ps lifetimes in air were reduced due to quenching by oxygen, so only results obtained under N2 are discussed here. Results were analysed in terms of a four component lifetime distribution, which allowed obtaining better statistical fit. The two longest lifetimes, T3 and T4, for PIM-1 in... 

For R6G on silica, experiments were performed on populations of molecules to obtain the mean, unperturbed lifetime, Zf, and its distribution. Five unperturbed fluorescence decay curves were measured for tips positioned 1.0 to 1.1 pm above high-coverage surfaces ( 10 to 10 molecules illuminated) (Fig. 11(D)). The decay curves were fitted to a single exponential with Zf = 3.65 + 0.04 ns, which is in good agreement with the 3.5+ 0.1 ns obtained previously [22]. Statistical noise and instrument nonlinearity place an upper limit on a possible Gaussian standard devia-... 

In practice, time-domain measurements can be done using time correlated single photon counting (TCSPC) whereby the arrival time of the first photon after each pulse is monitored at very high time resolution [ 17]. By recording the arrival times of a large number of photons, a representation of the decay curve is obtained. In order for this approach to work, the chance of detection of a photon after a pulse should be low. If this is not the case, the distribution will be biased toward shorter lifetimes. It has been estimated that, for TCSPC to work for lifetime measurements, the detection efficiency should be 1% or lower [15]. This means that TCSPC always is relatively slow. Advantage is that the actual decay curve is measured directly. 

Figure 12.10 Typical time traces of (a) emission intensityand (b) lifetime, measured from a single DMPBI nanocrystal, (c) Photon correlation histogram obtained from the time trace of the emission intensity (a). The lifetimes were obtained by fitting a single exponential function to the decay curves constructed for every 2000...

For fluorescence decay curves of the J-aggregate LB films of [CI-MC] mixed with various matrix agents, measured with a picosecond time-resolved single photon counting system, three components of the the lifetimes fitting to exponential terms in the following equation ... 

COLOR FIGURE 5.20 Temperature influence for fluorescence anisotropy measurements on C22-modified silica, (a) Decay curves for DPH at different temperatures, (b) schematic of wobble motion of DPH and resulting fluorescence lifetimes (rp) and half-cone angles (0). (Reproduced from Pursch, M., et al., J. Am. Chem. Soc., 121, 3201, 1999. With permission.)... 

ESE measurement at 4 K (Argonne National Laboratory). Numbers in parenthesis are the computed errors [(01) 1] from the ESE decay curves. The rate constants were derived from the reciprocal of the ESE measured lifetimes for the downfield y-component. 

Fig. 11.5 Measurement of lifetime of anthracene in solution by single photon time correlation technique. Fluorescence decay curve of 8 X10-4 M anthracene in cyclohexane in the absence (A) and presence (B) of 0.41 M CC14. Points experimental data Line best fitting single exponential decay convoluted with instrumental response function (C) Time scale 0.322 nsec per channel. (Ref. 13).

Figure 8. Fluorescence decay of Pr phytochrome (124 kDa) excitation at Aexc = 640 nm, emission measured at Aero — 680 nm. The semilogarithmic plots of the measured decay (curve with signal noise) and the decay function calculated from best-fit kinetics parameters obtained by single-decay analysis (thin line superimposed on measured decay) are shown. In the inset the calculated lifetimes xf 3 and relative amplitudes Rf 3 of the decay components are given. On top, a weighted residuals plot (sigma) indicates the deviations of these computer-fitted parameters from the measured decay, with the value of the squared reduced error (y2) in the inset. The fluorescence decay of the red-light adapted Pr + Pfr mixture exhibited a comparable triexponential behaviour. (After Figure 4 in Holzwarth et al. [76].)...

There are two cases of discrepancies outside of one standard deviation uncertainties l, B and lsBe. In both cases there was only one previous measurement and there is no obvious reason for the discrepancies.lt is possible that the background under the 1 "B decay curve of ref.[Alb74] was underestimated leading to the larger value of the extracted lifetime.In the case of the I5B lifetime of ref. [Duf] no decay curve was shown so it is hard to compare quality of fits. 

Up-conversion relies on sequential absorption and luminescence with intermediate steps to generate shorter wavelengths. Hence, the presence of more than one metastable excited state is required the intermediate metastable states act as excitation reservoirs. One typical example is ground-state absorption followed by inter-mediate-state excitation, excited-state absorption, and final-state excitation to give the up-conversion (the intermediate states and final states are real states) [1, 35], There are many types of up-conversion mechanisms such as excited-state absorption, energy transfer up-conversion and cooperative up-conversion. All these up-conversion processes can be differentiated by studying the energy dependence, lifetime decay curve, power dependence, and concentration dependence by experimental measurements [36-39]. 

Luminescence decay curves are also often used to verify that samples do not contain impurities. The absence of impurities can be established if the luminescence decay curve is exponential and if the spectrum does not change with time after pulsed excitation. However, in some cases, the luminescence decay curve can be nonexponential even if all of the luminescing solutes are chemically identical. This occurs for molecules with luminescence lifetimes that depend upon the local environment. In an amorphous matrix, there is a variation in solute luminescence lifetimes. Therefore, the luminescence decay curve can be used as a measure of the interaction of the solute with the solvent and as a probe of the micro-environment. Nag-Chaudhuri and Augenstein (10) used this technique in their studies of the phosphorescence of amino acids and proteins, and we have used it to study the effects of polymer matrices on the phosphorescence of aromatic hydrocarbons (ll). 

Time resolved A state fluorescence is measured with a transient recorder-signal averager, as described previously.(6J A typical fluorescence decay curve is shown in Figure 3. It is composed of at least three exponentials, independent of excitation wavelength. The short component lifetime is extracted by fitting approximately the first third of the decay to the expression. 

Two techniques, phase and pulse fluorometry, are used for the direct measurement of fluorescence decay rates, and their principles are described by Birks and Munro (1967), Parker (1968), and Birks (1970). The photon sampling method has proved useful and versatile. This is an iterative technique in which single photons are counted as a function of the time at which they appear after excitation and a complete decay curve is built up. (For recent references see e.g. Zimmerman et al., 1973, 1974). Wider use of the photon sampling technique will increase the precision of lifetimes obtained and extend the range of compounds studied to those with shorter lifetimes or very low fluorescence yields. 

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