Fluorescence delayer

Figure 35 Relative fluorescence efficiency as a function of the quantal exciting intensity for a 200 pm-thick tetracene crystal excited with the 325 nm line of a He-Cd laser. The increasing segment shows the triplet—triplet fusion contribution to the fluorescence (delayed fluorescence) the decrease at high excitation levels is attributed to quenching of the singlets by singlet-triplet annihilation. Experimental data are represented by points, theoretical fits, as described in text, by the solid line. Adapted from Ref. 206.

Fig. 3.28 Dependence of fluorescence, delayed fluorescence and phosphorescence intensity of PF2/6 in MTHF on pump intensity at 80 K. The time delay for measurements of DFand Ph was 1 ps. Excitation was at 3.05 eV Reprinted from [142], copyright 2002, with permission from the American Institute of Physics.

First excited singlet state Fluorescence, delayed fluorescence... 

Luminescence can be created by photoirradiation, which results in fluorescence, delayed fluorescence, and phosphorescence or by a chemical or biochemical reaction, which produces chemiluminescence and... 

Figure Al.6,8 shows the experimental results of Scherer et al of excitation of I2 using pairs of phase locked pulses. By the use of heterodyne detection, those authors were able to measure just the mterference contribution to the total excited-state fluorescence (i.e. the difference in excited-state population from the two units of population which would be prepared if there were no interference). The basic qualitative dependence on time delay and phase is the same as that predicted by the hannonic model significant interference is observed only at multiples of the excited-state vibrational frequency, and the relative phase of the two pulses detennines whether that interference is constructive or destructive.

CN] —> I + CN. Wavepacket moves and spreads in time, with its centre evolving about 5 A in 200 fs. Wavepacket dynamics refers to motion on the intennediate potential energy surface B. Reprinted from Williams S O and lime D G 1988 J. Phys. Chem.. 92 6648. (c) Calculated FTS signal (total fluorescence from state C) as a fiinction of the time delay between the first excitation pulse (A B) and the second excitation pulse (B -> C). Reprinted from Williams S O and Imre D G, as above. 

For fluorescent compounds and for times in die range of a tenth of a nanosecond to a hundred microseconds, two very successftd teclmiques have been used. One is die phase-shift teclmique. In this method the fluorescence is excited by light whose intensity is modulated sinusoidally at a frequency / chosen so its period is not too different from die expected lifetime. The fluorescent light is then also modulated at the same frequency but with a time delay. If the fluorescence decays exponentially, its phase is shifted by an angle A([) which is related to the mean life, i, of the excited state. The relationship is... 

Figure B2.3.9. Schematic diagram of an apparatus for laser fluorescence detection of reaction products. The dye laser is syncln-onized to fire a short delay after the excimer laser pulse, which is used to generate one of the reagents photolytically.

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... 

A dye molecule has one or more absorption bands in the visible region of the electromagnetic spectrum (approximately 350-700 nm). After absorbing photons, the electronically excited molecules transfer to a more stable (triplet) state, which eventually emits photons (fluoresces) at a longer wavelength (composing three-level system.) The delay allows an inverted population to build up. Sometimes there are more than three levels. For example, the europium complex (Figure 18.15) has a four-level system. 

Figure 9.43 The effect on time delay measurement of fluorescence intensity from sodium atoms in a transition state of Nal of changing the pump wavelength to (a) 300 nm, (b) 311 nm, (c) 321 nm, and (d) 339 nm. (Reproduced, with permission, from Rose, T. S., Rosker, M. J. and Zewail, A. H., J Chem. Phys., 91, 7415, 1989)...

The impurities may capture this migrating exciton and lose its excess energy. The mutual annihilation of two or more triplet excitons occurs in the same polymer chain and delayed fluorescence is observed. 

Emission of light due to an allowed electronic transition between excited and ground states having the same spin multiplicity, usually singlet. Lifetimes for such transitions are typically around 10 s. Originally it was believed that the onset of fluorescence was instantaneous (within 10 to lO-" s) with the onset of radiation but the discovery of delayed fluorescence (16), which arises from thermal excitation from the lowest triplet state to the first excited singlet state and has a lifetime comparable to that for phosphorescence, makes this an invalid criterion. Specialized terms such as photoluminescence, cathodoluminescence, anodoluminescence, radioluminescence, and Xray fluorescence sometimes are used to indicate the type of exciting radiation. 

Figure 1.5. Femtosecond spectroscopy of bimolecular collisions. The cartoon shown in (a illustrates how pump and probe pulses initiate and monitor the progress of H + COj->[HO. .. CO]->OH + CO collisions. The huild-up of OH product is recorded via the intensity of fluorescence excited hy the prohe laser as a function of pump-prohe time delay, as presented in (h). Potential energy curves governing the collision between excited Na atoms and Hj are given in (c) these show how the Na + H collision can proceed along two possible exit channels, leading either to formation of NaH + H or to Na + H by collisional energy exchange.

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