Luminescence decay curve

According to Ludwig (1968), there is a some similarity between UV- and high-energy-induced luminescence in liquids. In many cases (e.g., p-ter-phenyl in benzene), the luminescence decay times are similar and the quenching kinetics is also about the same. However, when a mM solution of p-terphenyl in cyclohexane was irradiated with a 1-ns pulse of 30-KeV X-rays, a long tail in the luminescence decay curve was obtained this tail is absent in the UV case. This has been explained in terms of excited states produced by ion neutralization, which make a certain contribution in the radiolysis case but not in the UV case (cf. Sect. 4.3). Note that the decay times obtained from the initial part of the decay are the same in the UV- and radiation-induced cases. Table 4.3 presents a brief list of luminescence lifetimes and quantum yields. 

Thus, the quantity b can be found from the luminescence intensity change at the instant of switching on the field, and further used for describing, with the help of eqn. (42), the luminescence decay curve when the field is on. 

Figure 4. Luminescence decay curve and its semilogarithmic representation for...

Fig. 9. Luminescence decay curves of Cs2NaYCl6 Sb3+ for different magnetic field strengths H. T= 1.9 K and H / / < 111 >. Reproduced with permission from Ref. [29]...

The high sensitivity and specificity of photoluminescence analysis should make it possible to individualize clue materials, e.g., hair and glass, by the characteristic luminescence properties of trace constituents or impurities. Of particular significance are the newer techniques of analyzing the luminescence decay curves. For example, even when the absorption and luminescence spectra of the impurities are similar, it is possible to determine their concentrations if their luminescence lifetimes differ. The usefulness of this technique is illustrated in Figs. 1 and 2, where it is shown that the fluorescence spectra of naphthalene (N) and 1,6-dimethyl napthalene (DMN) are too similar for fluorescence spectral analysis of their mixtures (Fig. l) yet their relative concentrations can be readily determined from the fluorescence decay curve (Fig. 2). As indicated by the dashed curve in Fig. 2, the observed decay is the sum of exponential decays from a shorter lived component, i.e., DMN (lifetime 50 nsec) and a longer lived component, i.e., N (lifetime 100 nsec). St. John and Winefordner (j) have discussed this technique in general and Hoerman and co-workers (8,9) have been... 

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). 

The preparation, characterization, aqueous stability, and photophysical properties of NIR emitting lanthanide complexes with tetradentate chelating ligands 36 and 37 were described by Raymond and coworkers [61, 62]. In aqueous solution, the chelating ligand 36 or 37 forms stable complexes with Ln(III) ions, and sensitized NIR lanthanide luminescence was detected for the complexes with Pr(III), Nd(III), Ho(III), or Yb(III) ions. For [Ln(36)2] complexes, the luminescence decay curves were biexponential due to partial hydrolysis of the complexes or alternately the presence of a slowly exchanging equilibrium mixture with a hydrated form of the complexes. For [Ho(37)2] , the NIR band due to Fs -> I transition of the Ho(III)... 

Luminescence decay curves may be observed by displaying the output of the photomultiplier on an oscilloscope. Precautions must be taken to correct for instrumental distortion of fast decay curves (D13). In multicomponent systems with differing decay times, electronic gating may be used to isolate the signal due to one component (time resolved phosphorimetry) (SI). A complete emission spectrum can be observed using a spectrograph with a photographic plate or television camera tube, but these systems are as yet only of specialist interest. 

Figure 2 shows luminescence decay curves for the a-Ti02 powder irradiated with NUV-LED both in air and in nitrogen. The peak luminous intensities measured in different atmospheres were the same within experimental error. One can see two steps in the decay curves. The half-life time of the first step was a few seconds... 

In order to confirm the adsorption of oxygen molecules on the a-Ti02 powder simultaneous photo- and heat-activated luminescence measurements were conducted with NUV-LED as a light sauce. Figure 7 summarized the results of luminescence decay curves of the a-Ti02 measured at temperatures ranging from 15 to 90 °C in air. Observed decay curves exhibited initial luminous intensity and the slope of the exponential decay curves became smaller with increasing temperature. 

Luminescence decay curves of both complexes excited with the 337 nm fundamental of the N2 laser at 77 K fit sufficiently well to a single exponential analysis. However, time resolution of the emission of Ir(ppy)2(bpy)+ via boxcar averaging techniques reveals a structured emission at long delay times... 

Details of the neutralization process following radiation-induced primary charge separation may be examined via the medium of ultrafast techniques now employed in studies of luminescence decay processes. As an example, the form of luminescence decay curves of dilute organic scintillator in aliphatic hydrocarbon solution excited by x-ray pulses of about 0.5-1.0 nsec, duration is attributed (in previous papers) to neutralization processes involving ions. The relation, t cc r3, for the time required for neutralization of an ion pair of initial separation r, when applied to such curves, leads to a distribution function of ion-pair separations. A more appropriate and desirable approach involves solution of a diffusion equation (which includes a Coulomb interaction term) for various initial conditions. Such solutions are obtained by computer techniques employed in analogy to corresponding electrical networks. The results indicate that the tocr3 law affords a fair description of the decay if the initial distribution can be assumed to be broad. 

The main features of this experiment which require an explanation are the complex shape of the luminescence decay curves, especially at 93°K., and the temperature dependence of the emission. On a simple excitation theory which will be considered initially, only triplet states remain 1 /xsec. or longer after irradiation as all the singlets produced will have returned to the ground state or undergone intersystem crossing to triplet states. In addition to the direct phosphorescence, Ti — S0, a number of different pathways are available for the removal of the remaining triplets. A complete decay scheme must include the following processes. 

However, there is a very usefiil reason for determining the type of luminescence decay curve present. Confirming the presence of an exponential decay curve means that only one type of emitter is jn sent. If a logarithmic decay process is found, it usually means that more than one type of emitting center is present, or that two or more decay processes are operative. While this does not occur very often, it is useful to know if such is present. This phenomenon occurs more in cathode-ray phosphors than in lamp phosphors, i.e.- sulfides vs oxide- hosts. 

Several hetero-bischelated complexes of Ir(III) with 1,10-phenanthroline and substituted 1,10-phenanthroline have also been reported to have non-exponential luminescence decay curves (19). Although the individual emission spectra of the non-equilibrated levels of these complexes are again too close to resolve by conventional emission spectroscopy, partial resolution has been accomplished by time-resolved emission spectroscopy via box-car averaging techniques (20). Complete resolution has been accomplished by computer analysis of luminescence decay curves as a function of emission wavelength (20). In these complexes, the luminescent levels appear to arise from both ligand-localized ( tttt ) states and charge-transfer ( ) states. 

The hetero-bischelated complex, [IrCl2,phen) (5,6-mephen)]Cl, displays a non-exponentital luminescence decay curve when excited at 337 nm in ethanol-methanol glass at 77°K (19), Analysis of the decay curves of this complex by a non-linear least squares fit to a function representing the sum of two exponentials indicates that the emission is caused by levels with lifetimes of 65 and 9.5 /msec (20), Both time-resolved spectroscopy and analysis of decay curves as a function of emission wavelength indicate... 

The complex [IrCl2(phen) (4,7-mephen)]Cl also displays a nonexponential luminescence decay when excited at 337 nm in ethanol-methanol glass at 77°K (20). Analysis of the luminescence decay curves of this complex by a two-state model indicates that the lifetimes of the two manifolds are 22 and 8.8 / sec. In contrast to the previous complex, it is the short-lived manifold of levels which lie lowest in this molecule. The splitting of the two manifolds of levels is about 300 cm" in this case. 

The luminescence decay curves were registered on a SPEX Fluorolog F212 spectro-fluorimeter linked to a 1934 D phosphorimeter with a 150 W pulsed xenon lamp. 

Fig. 5. Luminescence at 700 nm (dotted curve) plotted with the square of that at 1270 ran (solid). The 700 ran luminescence decay curve was normalized to the same scale as the 1270 nm. 

Fig. 27 Comparison of 60 and DAPI for resistance to photobleaching. (a) Confocal luminescence images of fixed HeLa cells stained with 60 and DAPI under continuous excitation at 405 nm with different laser scan times (0, 200, 480 s). (b) Luminescence decay curves of 60 and DAPI during the same period. The signals of DAPI and 60 were collected from region 1 of channel 1 (460 20 nm) and region 2 of channel 2 (620 20 nm), respectively...

Hgure 2 Source and luminescence decay curves 1, source time profile 2, fluorescence decay curve 3, true fluorescence decay curve. Assuming (j> the initial part of the decay cunre is distorted. 

The D-A transfer can speed up the decay of D in its excited state and thus shorten the decay time of D. As a result, r/g can also be obtained from the decay curve of the D luminescence after flash excitation of D. The decay curve represents the decay of transient luminescence intensity, which can be regarded as the number of photons emitted per time at time t. Denote the D-luminescence decay function by Io(t) for the absence of A and by I t) for the presence of A. The decay function can be obtained by normalizing the initial intensity of the experimentally measured luminescence decay curve, meaning 7o(0) = 7(0) = 1. The integrals of the intensity-decay function over time are proportional to the number of total photons emitted after excitation, i.e., it is just the measure of the steady-state luminescence intensity. From Eqs. (3.1) and (3.2), one thus has [4]... 

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