Lifetime difference, measurable

Luminescence lifetimes are measured by analyzing the rate of emission decay after pulsed excitation or by analyzing the phase shift and demodulation of emission from chromophores excited by an amplitude-modulated light source. Improvements in this type of instrumentation now allow luminescence lifetimes to be routinely measured accurately to nanosecond resolution, and there are increasing reports of picosecond resolution. In addition, several individual lifetimes can be resolved from a mixture of chromophores, allowing identification of different components that might have almost identical absorption and emission features. 

A mixture of noninteracting fluorophores might be observed by spatially variant FRET in a specimen, which is blurred because of optical resolution issues, will result in different lifetimes being measured for zm and zy- and zm > %f. In many instances, a single frequency measurement will be insufficient to determine the number of fluorophores or the number of fluorophore environments in a sample. 

Our observation of the hydrated electron band at a 5 /xsec. delay cannot be attributed to the thermal reaction H + OH - e aq + H20, because the rate constant of 1.8 X 107Af 1 sec. 1 (21) permits only a negligible conversion of H atoms below pH 10. Therefore, the cases indicated as (+) and (+ + ) are taken as definite proof of photoionization. The cases indicated as (f) are less certain, although a photographic density difference of proper lifetime was measured densitometrically because the weak absorptions made the delineation from other transients, such as short-lived triplets, less certain. The absence of the hydrated electron... 

Different situations can be found direct determinations, where the lu-minophore is the analyte itself, or indirect determinations, where the analyte interacts with a luminophore, changing its optical properties, the quenching effect. And different measurement possibilities can be followed intensity or lifetime of emission. 

Two very important points come from the above discussion. The first is that both the observation area and the cell size must be considered. If excited molecules are lost from view, it is essentially equivalent to them being quenched on the walls. The only difference is that they may diffuse back into the viewing area however, there is still a net loss of fluorescence signal. The second point is that the timebase of the experiment is of equal importance to the lifetime being measured. Obviously, it is better to measure the decay over as large a dynamic range as possible, but in order to get an accurate measurement of the lifetime a compromise may be necessary. 

The value of kd was obtained from the determination of triplet lifetimes by measuring the decay of phosphorescence and found to be insensitive to changes in solvent polarity. The k2 values derived from Eqs. 10 and 11 were correlated with solvent parameters using the linear solvation energy relationship described by Abraham, Kamlet and Taft and co-workers [18] (Eq. 12), which relates rate constants (k) to four different solvation parameters (1) or the square of the Hildebrand solubility parameter (solvent cohesive energy density), (2) n or solvent dipolarity or polarizability, (3) a, or solvent hydrogen bond donor acidity (solvent electrophilic assistance), and (4) or solvent hydrogen bond acceptor basicity (solvent nucleophilic assistance). 

When a thermal beam or a laser-produced plasma is applied, it is much more difficult to evaluate concentrations. Then the lifetimes are measured at different experimental conditions in order to observe possible influences of collisions on the measured lifetimes see Fig. 9... 

BOj (v = 40, J = 77). Lifetimes are measured by a phase-shift technique (the excitation radiation is amplitude modulated at 100 kHz) and the arrow points in the direction of increasing lifetime. Note that the difference vanishes at if = 0 and reverses between if > 0 and... 

Positron lifetime spectroscopy has been shown to be a good means of investigating the structural levels of nanocrystalline materials [48]. Different annihilation sites (dislocations, micropores, and mesopores) have been attributed to the different measured positron lifetimes. 

Table 2.5 yields the different values of the mean fluorescence lifetime and of intensity of fluorescein in presence of increased concentrations of KI. Lifetimes were measured with both frequency domain and Time correlated single photon counting methods. Figure 2.20 displays the normalized values at different Kl concentrations. 

The generation lifetime is measured with the pulsed MOS capacitor or the gate-controlled diode technique. It is important to understand that the lifetime measured by this technique can, and generally does, give very different values from the recombination lifetime measured by one of the techniques indicated above. 

For closely spaced lifetimes, the ASEs will greatly underestimate the uncertainties in the parameters. This underestimation of errors is also illustrated in Ihble 4.7, which lists the analysis of the three-component mixture when measured at various emission wavelengths. It is clear from these analyses that the recovered lifetimes differ by amounts conriderably laager ttian Ae ASEs. This is par-dcularly true for the ftactional intensities, for which the ASEs are 0.001. Similar results can be expected fx any decsy with closdy spaced Ufedmes. 

LIBS-LIF spectra are very different from usual LIBS spectra (Fig. 6.9). Estimated LIF decay time under VIS excitation is as rapid as the excitation OPO pulse width of 4 ns. The cause of such emission decay time shortening may be collisional quenching of the molecular excited states in LIP or thermal quenching of the excited states at high plasma temperature. Due to such short emission lifetime, MLIF measurements were done with a short gate width of W= 10 ns a delay of... 

However, all the authors mentioned a rapid decrease of the luminescence intensity with further heating above the threshold temperature, which was attributed to thermal decomposition of the complex. Among the different tested complexes, it seems that phenanthroline exhibited best thermal stability, because no luminescence intensity decrease was observed up to ISO C, which allowed more efficient water elimination. Accordingly, best lifetime values measured on films doped with europium complex were reported by Li et al. for complexation with phenanthroline (Li et al., 2001). The lifetime characterized as the first e-folding decay time was measured to be 1.40 ms. However, these authors also mentioned a bi-exponential luminescence decay, which indicated that Eu " ions occupy two kinds of spectroscopic sites. This feature probably indicates that all ions were not similarly encapsulated in the phenanthroline cage. 

A simplified theory was proposed by Brandt, Berko and Walker [104] in which the positron of Ps wave function in the field of the electron was replaced by the wave function of the Ps atom. The Ps wave function was then calculated for different lattice structures in the Wigner-Seitz approximation. This approximation is generally referred to as the free volume model, since the free volume is used as one of the parameters in the calculation. This model relates o-Ps lifetime to the average free volume hole size of the medium, and results construed that the o-Ps lifetime would measure the lattice-Ps interaction. Later, Tabata et al. [105] and Ogata and Tao [106] each adopted similar - but different - approaches by considering a unit cell and Ps located at the center instead of the center of the molecule, as used by Brandt et al. [104]. 

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