Fluorescence fast component

This behavior is consistent with experimental data. For high-frequency excitation, no fluorescence rise-time and a biexponential decay is seen. The lack of rise-time corresponds to a very fast internal conversion, which is seen in the trajectory calculation. The biexponential decay indicates two mechanisms, a fast component due to direct crossing (not seen in the trajectory calculation but would be the result for other starting conditions) and a slow component that samples the excited-state minima (as seen in the tiajectory). Long wavelength excitation, in contrast, leads to an observable rise time and monoexponential decay. This corresponds to the dominance of the slow component, and more time spent on the upper surface. 

The accuracy with which a system can measure lifetimes depends on a number of different factors including calibration of the instrument, the number of detected photons and also the efficiency of the analysis routines. In addition, sources of background and scattered light should be eliminated. Emission filters should be chosen with great care to make sure that no scattered laser light reaches the detector. Detection of scattered excitation light results in a spurious fast component in the decay and complicates the interpretation of the data. The choice of emission filters is much more critical in FLIM than in conventional fluorescence intensity imaging methods. 

Lifetime heterogeneity can be analyzed by fitting the fluorescence decays with appropriate model function (e.g., multiexponential, stretched exponential, and power-like models) [39], This, however, always requires the use of additional fitting parameters and a significantly higher number of photons should be collected to obtain meaningful results. For instance, two lifetime decays with time constants of 2 ns, 4 ns and a fractional contribution of the fast component of 10%, requires about 400,000 photons to be resolved at 5% confidence [33],... 

The values for k+ and k were compared for temperature jump and stopped-flow conditions for DNA concentrations where the decay followed a mono-exponential function and no migration between DNA molecules occurred (see below).94 This report shows the importance of detecting fluorescence signals at the magic angle, which eliminated the fast components in the kinetics due to artifacts. The values for the association and dissociation rate constants obtained by both techniques are similar. 

To avoid depolarization by excitation transfer, the DNA is unwound using a second intercalator, for example, chloroquine, that does not engage in excitation transfer to or from the extremely dilute FPA probe, ethidium. Equation (4.68) applies to chloroquine when it is in excess, but the simultaneous binding of trace ethidium obeys a somewhat different relation, which is expressed in terms of the ratio of amplitudes (Ab/Af) of the bound (slow) and free (fast) components in its fluorescence decay as follows<53) ... 

Figure 5-9. TRSEP signal for the aniline(N2)i cluster. Excitation laser is tuned to the TJj transition, and the probe laser is tuned to the I 6a transition. This plot shows the extent to which the probe pulse diminishes the total fluoresence. The time axis is the difference between the arrival times of the pump and probe pulses. The maximum diminution of the fluorescence is about 30%. The smooth curve is generated using the results of a nonlinear fitting routine. The fast component time constant is 200 + 50 ps.

Phosphorescence of s-triazine has been observed by Ohta et al. following excitation of the 6o band of the Si — So transition. Values for the phosphorescence lifetime and quantum yield were reported. The effects of rotational excitation on the yields and decays of the fast and slow components of Si state s-triazine fluorescence have been studied. Excitation along the rotational contours of the 6j and 6o bands revealed that the fast component showed little rotational level dependence in contrast to the slow component. This behaviour was interpreted in terms of an increase in the number of triplet levels coupled to the optically prepared singlet levels with increasing angular momentum quantum number, J. A broad emission feature present in addition to narrowline fluorescence from rovibronic levels of 6 or 6 in S, s-triazine has been observed and the rotational level dependence of its quantum yield and decay over a range of pressures reported... 

Energy migration may reveal itself as a fast component in the decay of fluorescence of M, being often in the pfcosecond region, but possibly in some polymer systems on the nanosecond time scale. The phenomenon can certainly contribute greatly to the observed depolarization of fluorescence ... 

From fluorescence depolarization measurements, anisotropy relaxation times and the associated anisotropy values have been determined for p-C2P1 p-C2P2, p-C2P3, and / -C2P. For the dendrimers with more than one chromophore, a two-exponen-tial function was found to be necessary to fit the experimental anisotropy decay traces (Table 1.2). The multichromophoric dendrimers present two-exponential decays in the anisotropy traces. The fast component (410 ps to 280 ps) of the anisotropy decay (Table 1.2) is found to decrease from p-C2P2 to p-C2P4. Contrary to the meta-substituted dendrimers m-C 1 P , the sum of the / , is now always close to the limiting value of the anisotropy even if 11 is larger than one. 

The shorter decay (instrument-limited) was attributed to the fluorescence from adsorbred molecules strongly coupled to the surface, indicating that the electron transfer rate is much faster than 40 ps. In order to get a more accurate value for the injection rate, the integrated fluorescence intensities of the fast component of the sensitized semiconductor decay curve were compared with the intensity of the oxazine on the tape reference sample [48]. This analysis yielded a quenching factor of about 10 . From this an electron injection rate of 3 x lO s corresponding to an electron transfer time of around 40 fs, was obtained. 

In a recent paper (12) we showed that the species responsible for the green (543 nm) fluorescence of 3HF was formed kinetically from blue (413 nm) emitting species by two kinetic components. We rationalized this complex behavior by a model (Scheme 1) in which there are two pathways for excited state isomerization, i.e., a slow and a fast component. 

We attributed also to injection the dominant fast component of the fluorescence on the semiconductors. The minor slow component was attributed to molecules adsorbed at a few semiconductor surface sites where injection cannot take place. The rate constants kg and k were estimated from the data obtained at the lowest concentration (10 M), so as to minimize energy transfer effects. The values = r2(dry) = 3.1 ns and kj = 0 were used in [5] to derive kg = 3.2 x 10 s. This kg value and Tf = Ti were then used in [5] and in [7] to derive kj = 2.1 x 10 s and = 0.88 both for Sn02 and for InjOj. This kj value is six times lower than that estimated from our steady-state measurements for In203 and three times higher than calculated by Itoh et al. (16) for Sn02. Because the time-resolved measurements are much more direct, we consider this kj value to be more reliable than those derived from steady-state measurements. 

These contradictory results led Jonkman et al.21 to propose that non-resonant light scattering (NRLS) was responsible for the fast component. NRLS would yield decays that basically consisted of Raman-Rayleigh-scattered laser light together with the slower fluorescence decay. It would look like biexponential decay. Experiments where the laser was purposely detuned from the rotational line seemed to confirm their ideas.21... 

Figure 19. Double exponential fit (line) to the measured decay of Fig. 18 bottom. The weighted difference (residual) between the observed decay and the fitted curve appears at the top of the figure. Best-fit values of the two lifetimes and the ratio of fast to slow fluorescence are given in the figure. One notes that the best-fit fast-to-slow ratio is greater than what one would judge by eye from the decay because the finite temporal response of detection tends to reduce the apparent magnitude of the fast component.

Picosecond laser measurements with trans-stilbene in solution (and even in the gas phase) agree in the respect that the fluorescence decay is single exponential (Table 14). Earlier results assuming a slow and a fast fluorescence decay component [376] have not been confirmed in at least four different laboratories [314, 321-323, 342, 349]. Because of the monoexponential decay, the back reaction 1 p —> 11 has to be much slower than the forward reaction t -. Elowever, the adiabatic formation of t from c indicates that the rate constants of both processes in solution have the same order of magnitude, independent of temperature. The low efficiency, %0.3%, of the p -> t process reflects very rapid p - p decay rather than a low p energy [81]. 

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