Decay curves spectra

Figure 2.7. The fluorescence spectra from unrelaxed (/ = 0) and relaxed ( - oo) states and the emission decay curves at the short-wavelength edge (a), the maximum (b), and the long-wavelength edge (c) of the spectrum.

Luminescence of in synthetic alkaline earth sulfates is well known (Folk-erts et al. 1995). In this study, CaS04 Pb shows an emission band with a maximum at 235 nm at 300 K, while the excitation maximum is at 220 nm. The decay curve of the emission is single exponential with a decay time of 570 ps at 4.2 K. The emission spectrum of BaS04 Pb demonstrates a broad band peaking at 340 nm with an excitation maximum at 220 nm, while in SrS04 Pb the luminescence band has a maximum at 380 nm. hi natural barite and anhydrite samples we detected several narrow UV bands, which may be connected with Pb emission, but for confident conclusion additional study is needed. In any case, Pb participation in natural sulfates liuninescence has to be taken into consideration. 

Weller24 has estimated enthalpies of exciplex formation from the energy separation vg, — i>5 ax of the molecular 0"-0 and exciplex fluorescence maximum using the appropriate form of Eq. (27) with ER assumed to have the value found for pyrene despite the doubtful validity of this approximation the values listed for AHa in Table VI are sufficiently low to permit exciplex dissociation during its radiative lifetime and the total emission spectrum of these systems may be expected to vary with temperature in the manner described above for one-component systems. This has recently been confirmed by Knibbe, Rehm, and Weller30 who obtain the enthalpies and entropies of photoassociation of the donor-acceptor pairs listed in Table XI. From a detailed analysis of the fluorescence decay curves for the perylene-diethyl-aniline system in benzene, Ware and Richter34 find that... 

In Fig. 3, the oscillating parts extracted from the fluorescence decay curves of the w.-t. PYP at blue and red edges of the spectrum are shown, which is in accordance with the simplified schematic image of the Fl dynamics of Fig. 2. 

We further carried out time-resolved fluorescence measurement on a single fluorescent spot of the individual enzyme-TNP-ATP complexes. Figure 24A and B shows a representative fluorescence decay curve and fluorescence spectrums of single enzyme-TNP-ATP complexes. The fluorescence spectrum varied from spot to spot, as an indication of fluorescence from individual complexes. The decay curve cannot be fitted to a single exponential, but was well fitted to a... 

TNP-ATP complex obtained by the single-molecule time-resolved spectroscopy, together with a fluorescence decay curve of TNP-ATP obtained by a bulk measurement. Both curves were well fitted to biexponential functions. The instrument-response function in 195-ps fwhm is also displayed. (B) Representative fluorescence spectrums of two individual enzyme-TNP-ATP complexes showing different emission peaks. A fluorescence spectrum of TNP-ATP obtained from a bulk measurement is also displayed for comparison. All spectrums were normalized to unity at their maximum. (From Ref. 18.)... 

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). 

Transverse relaxation is caused by the distribution and fluctuation of the resonance frequency of the A spins. The distribution-induced relaxation is called free induction decay. The free induction decay curve is the Fourier transform of the spectral shape of the A spins. This spectral shape depends on the intensity and the pulse width of the incident microwave, when the total width of ESR spectrum is large as is the case for radical species in solids. Therefore, the analysis of the free induction decay curve gives no information on the nature of radical species in solids unless the pulse width is narrow enough to cover the entire ESR spectrum. 

A typical example is as follows Molecule 7h was studied in the flash photolysis apparatus.42 67 A solution of 7h was irradiated at varying wavelengths between 375 and 700 nm. From the 19 decay curves registered, the spectrum of the transient 8h was determined (see Figures 6.15 and 6.16). 

Decay curves of PP radicals are shown in Fig. 12. The solid curve in Fig. 12 is a decay curve of the If mechano-radical produced by the sawing and tl dotted line is that of the PP radical produced by 7-iiradiation. The radicals i oduced by 7-irradiation decay step-wise with increasing temperature of the heat treatments, and no increase of the ESR intensity is observed as a result of the heat treatments. This is common and normal behavior in the dec y of polymer radicals (79-81). Contrastly the decay curve of the mechano-radical increases to a maximum at 173 K and decreases more rapidly than that by 7-irradiation in the higher temperature region. It was also found that the ESR spectra changed in shape during this anomalous increase of the intensity, as shown in Fig. 13. The ESR spectrum observed before the heat treatment was... 

The most important parameters of the photolumincsccnce spectrum which are necessary to identify the emitting species are its spectral shape, from which the emission intensity 4 is obtained as a function of wavelength A its quantum efficiency e, by which the intensity is measured relative to the absorption photointensity /a and its lifetime t, determined from the decay curves of variation of the intensity measured as a function of time (33, 34, 36-38, 49-53). 

To obtain a tme spectrum, I(t) is actually measured at a number of wavelengths and G(t) obtained by deconvolution Then the individual G(t)s are scaled to the intensity in the total spectmm and photons in the derived time slice are summed and arranged as a function of wavelength. This method will be referred to as Method II. Spectral resolution is now determined by the number of wavelengths at which I t) was measured while time resolution is limited only by the channel width in the decay curve measurement. 

In Fig. 13 are shown the rectra, measured from separate solutions of MOI and AMP in water. The lifetimes of MOI and AMP, measured by fitting decay curves from the separate solutions to single exponential functions, were 4.48 ns and 9.57 ns respectively. Therefore the ectrum obtained from a mixture of MOI and AMP in water presented in Fig. 14 ould be resolvable into die two spectra of Fig. 13 using Method I. The TRES depicted in Fig. IS illustrate this separation. It will be seen that the EGS diows a broadening to the red compared to the MOI spectrum while the LGS is broader to the blue than the AMP qiectrum. This is expected in view of the fact that AMP makes some contribution to the emission even at extreme diort times while at a At of 54 ns there is still some fluorescence from MOI. These ectra clearly demonstrate that TRES collected in this way are a good diagnostic tod in examining complex spectra but will not completely separate the individual components when the lifetimes of each species are not very different. 

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. 

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