Delayed emission, lifetime measurement

Some degree of temporal resolution of emission may be obtained by incorporating a phosphoroscope attachment in the simple apparatus described above. A mechanical or electronic device is used to allow periodic and out-of-phase excitation and detection of luminescence. In the simplest case a mechanical shutter interrupts the excitation beam periodically and the detection system is gated so that emission is observed only after a fixed interval of time has elapsed after excitation. Under these conditions short-lived processes such as prompt fluorescence will have decayed to zero intensity and only longer-lived emission will be recorded. For mechanical devices the limit of measurable lifetime is of the order of 1 ms, thus allowing time resolved studies to be made of certain phosphorescence and delayed emission procesres (see ... 

In the oxidized sample, a prompt Chl-a fluorescence with a 0.42 ns lifetime is seen [Fig. 3 (A), trace (a)]. In the sample maintained at -450 mV and illuminated to photoaccumulate O ", the prompt fluorescence lifetime is 0.18 ns [trace (b)]. In the sample maintained at -450 mV but kept in the dark, the signal shows a prompt and a delayed fluorescence (or delayed light emission) with lifetimes of 1.06 ns and 4.3 ns, respectively [trace (c)]. Note that in the extended time region [panel (B)], only trace (c) still has a measurable emission tail. These results show that neither the sample in the oxidized state nor that with O" photoaccumulated show any delayed light emission. Only the sample in which Qa is pre-reduced shows the A3-ns luminescence. These results support the notion that the delayed emission is a recombination luminescence originating from the [P680 <1)"] state. 

Intensity and lifetime measurement of BrQBA. 100 pL BrQBA ethanol stock solution of 5.0 mmol/L and an appropriate volume of NaDC solution were transferred into a 10 mL comparison tube, and mixed and finally diluted to 10 mL. The excitation and emission slits were both set at 20 nm. The delay time and gate time were set at 0.10 and 5.0 ms for intensity measurement, and both at 0.1 ms for lifetime measurement. 

A more direct method for lifetime measurements is the delayed coincidence technique [6] in which the time between an initiation event and the emission of a decay product is measured. A schematic diagram of an apparatus used for the measurement of atomic lifetimes is shown in figure BLIP.5. The slope of the graph of the natural log of the number of decay events as a function of time delay gives the lifetime directly. The... 

Emission lifetimes for spin-allowed d-d transitions of coordination compounds apparently have not yet been measured under photochemical conditions. The radiative lifetime, t , can be estimated (with serious potential error) from the area under the absorption band (9) the values are in the order of microseconds. The low-temperature fluorescence lifetime of Cr(urea)e was reported as 50 /xsec (9, 19). If one assumes that Enr averages about 3 kcal/mole, an order of magnitude calculation suggests a room temperature lifetime of about 0.02 nsec, which is still long compared with vibrational periods. This may be delayed fluores-... 

Phase-resolved, phase-modulation, or phase-sensitive lifetime measurements are based on the use of a continuous, sinusoidally modulated excitation source and phase-sensitive detection (Figure 7). The experimentally measured parameters are the modulation (m) and the frequency-dependent phase shift (4 ). The modulation of the excitation is given by bla, where a is the average intensity and b is the modulated amplitude of the incident light. For emission, the modulation is similarly defined, except using the intensities of the emission, BM, relative to the modulation of the excitation, m = B/A)/ b/a). The phase delay or phase angle ( P) is usually measured from the zero-crossing time of the modulated components. For an exponential decay, the fluorescence lifetime Tf can be calculated from the phase shift or... 

Luminescence lifetimes measured using pulsed laser excitation involve either direct detection of emission decays with time or a technique known as time-correlated single photon counting (TCSPC). The latter technique involves repeated measurement of the delay time between the excitation pulse and the arrival of an emitted photon packet above a given discrimination level the intensity-time decay profile accumulates over many millions of excitation pulses. The TCSPC experiment has the advantage that much better signal to noise can be obtained relative to the direct capture of the luminescence decay. 

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... 

Molecular Phosphorescence Instrumentation for molecular phosphorescence must discriminate between phosphorescence and fluorescence. Since the lifetime for fluorescence is much shorter than that for phosphorescence, discrimination is easily achieved by incorporating a delay between exciting and measuring phosphorescent emission. A typical instrumental design is shown in Figure 10.46. As shown... 

In TG methods, the fluorescence emission is detected in two or more time-gates each delayed by a different time relative to the excitation pulse (see Fig. 3.3). In the case of a detection scheme equipped with two time-gates, the ratio of the signals acquired in the two time-gates is a measure of the fluorescence lifetime. For a decay that exhibits only a single exponent, the fluorescence lifetime is given by ... 

In phase-fluorimetric oxygen sensors, active elements are excited with periodically modulated light, and changes in fluorescence phase characteristics are measured. The delay or emission (phase shift, ( ), measured in degrees angle) relates to the lifetime of the dye (x) and oxygen concentration as follows ... 

Prior to describing the possible applications of laser-diode fluorometry, it is important to understand the two methods now used to measure fluorescence lifetimes these being the time-domain (Tl)/4 5 24 and frequency-domain (FD) or phase-modulation methods.(25) In TD fluorometry, the sample is excited by a pulse of light followed by measurement of the time-dependent intensity. In FD fluorometry, the sample is excited with amplitude-modulated light. The lifetime can be found from the phase angle delay and demodulation of the emission relative to the modulated incident light. We do not wish to fuel the debate of TD versus FD methods, but it is clear that phase and modulation measurements can be performed with simple and low cost instrumentation, and can provide excellent accuracy with short data acquisition times. 

The time-resolved emission spectra (TRES) and fluorescence lifetimes, ti, of the fluorene derivatives were measured in liquid solutions at room temperature with a PTI QuantaMaster spectrofluorimeter with 0.1 ns temporal resolution [20]. At this resolution, all investigated fluorenes exhibited TRES which were coincident with the corresponding steady-state fluorescence spectra. As an example, TRES for compounds 3 and 11 in hexane, THE, and ACN are presented in Eig. 8 for different nanosecond delays 0 ns (curves 2,4,6) and 5 ns, which modeled the steady-state condition (curves 3,5,7). No differences in the fluorescence spectra for these two delays were observed, indicating that all relaxation processes in the first excited state Si are sufficiently fast for fluorene molecifles and did not exceed the time resolution of the PTI system ( 0.1 ns). 

The simple triplet-triplet quenching mechanism requires that at low rates of light absorption the intensity of delayed fluorescence should decay exponentially with a lifetime equal to one-half of that of the triplet in the same solution. Exponential decay of delayed fluorescence was, in fact, found with anthracene, naphthalene, and pyrene, but with these compounds the intensity of triplet-singlet emission in fluid solution was too weak to permit measurement of its lifetime. Preliminary measurements with ethanolic phenanthrene solutions at various temperatures indicated that the lifetime of delayed flourescence was at least approximately equal to one-half of the lifetime of the triplet-singlet emission.38 More recent measurements suggest that this rule is not obeyed under all conditions. In some solutions more rapid rates of decay of delayed fluorescence have been observed.64 Sufficient data have not been accumulated to advance a specific mechanism but it is suspected that the effect may be due to the formation of ionic species as a result of the interaction of the energetic phenanthrene triplets, and the subsequent reaction of the ions with the solvent and/or each other to produce excited singlet mole-... 

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