Fluorescence relaxation

Maurer s analysis of the nonexponential decays is of importance for it gives some insight into fluorescent relaxations in glasses. He postulates that there is a variation of transition probabilities as a result of a distribution... 

The dynamics of the fluorescence relaxation shift of the dual fluorescence-nitroxide probe... 

Figure 16.5 Processes involve in the fluorescence relaxation pathways of metal / organic optoelectronic devices.

This inactivation could neither be localized at the water splitting enzyme nor at photosystem I which both did not contribute to the observed Inactivation as tested by DPC to MV and DAD to MV Hill-reactions. Fluorescence relaxation studies confirmed that photoinhibition at 0 C led to an impairment of the QA -reoxidation as can be concluded from the slower decline of fluorescence following saturating single turnover flashes (Fig.2) (31. This implies a partial inhibition of the electron transfer through the Qs-binding site. 

FIGURE 2. Fluorescence relaxation kinetics following saturating single turnover flashes of thylakoids photolnhibited at 0 C (a) and measured at 20 C compared to a dark control (b). 

QA- reoxydation kinetic can be studied by chlorophyll fluorescence leaf relaxation after illumination by saturating light. Resistant biotype Solanum nigrum has been characterized by a clear decrease of fluorescence relaxation rate. That indicates an inhibition of electron transfer... 

Kinetics of chlorophyll fluorescence relaxation induced by a single saturating flash in atrazine R and S Solanum nigrum leaves. 

FIGURE 18-13 Energy diagram for (a) resonance Raman scattering and (b) fluorescence emission. Radiationless relaxation is shown as wavy arrows. In the resonance Raman case, the excited electron immediately relaxes into a vibrational level of the ground electronic state giving up a Stokes photon v,. In fluorescence, relaxation to the lowest vibrational level of the excited electronic state occurs prior to emission. Resonance Raman scattering is neariy instantaneous, and the spectral bands are very narrow. Fluorescence emission usually takes place on the nanosecond time scale. Fluorescence spectra are usually broad because of the many vibrational states. 

A Stack of 500x400 pixels x 9 planes was acquired every second over a 2-niin period with only 50 ms exposure time per plane. The acquisition of each stack in z-streaming mode took less than 500 ms, allowing more than 500 ms for fluorescence relaxation. Figure 20 shows the measured speedup factor as a fimction of the number of remote servers. The speedup factor is defined as the time required to restore all the stacks in sequential mode, divided by the time measured to restore all the stacks in parallel mode. The only difference between the algorithms is the computing time per stack. 

Colas des Francs, G., Bouhelier, A, Finot, E., Weeber, J. C., Dereux, A., Girard, C, and Dujardin, E. (2008) Fluorescence relaxation in the nearfield of a mesoscopic metallic particle distance dependence and role of plasmon modes. Opt. Express, 16,17654-17666. 

Figure 6.4 Stopped-flow/fluorescence relaxation signal from a 1.2 mM soybean lecithin-0.1 mM cetyltrimethylammonium bromide mixed with 1.2 mM lecithin-0.01 mM cetylpyridinium chloride. Both solutions also contained 0.15 M NaCl, 5 vol.% ethanol and 15 pM pyrene. The signal is biphasic (see break in time scale). The fast process corresponds to the transfer of pyrene between vesicles. The slow process is associated with the motion flip-flop of the cetylpyridinium chloride. Reproduced from Reference 42 with permission of the American Chemical Society.

The estimated millisecond time scale for exchange cannot be due to a helix-to-coil transition because for hairpin loops lifetimes of 10- 1(X)/is are expected (no Mg + in 30 mAf Na" "). Robillard et al. (1977), Romer et al. (1970), and Coutts et al. (1975) have shown that disruption of the teritary structure is associated with a longer (2-23 ms) relaxation time. More recently, Labuda and Porschke (1980, 1982), using temperature-jump Y-base fluorescence relaxation kinetics, have identihed a conformational transition in the anticodon loop of tRNA in a similar Mg + buffer. Their measured relaxation time of 1 ms at 7°C is, however, shorter than our rate process. 

Loring R F, Van Y J and Mukamel S 1987 Time-resolved fluorescence and hole-burning line shapes of solvated molecules longitudinal dielectric relaxation and vibrational dynamics J. Chem. Phys. 87 5840-57... 

Murakami H, Kinoshita S, Hirata Y, Okada T and Mataga N 1992 Transient hole-burning and time-resolved fluorescence spectra of dye molecules in solution evidence for ground-state relaxation and hole-filling effect J. Chem. Phys. 97 7881-8... 

A different example of non-adiabatic effects is found in the absorption spectrum of pyrazine [171,172]. In this spectrum, the, Si state is a weak structured band, whereas the S2 state is an intense broad, fairly featureless band. Importantly, the fluorescence lifetime is seen fo dramatically decrease in fhe energy region of the 82 band. There is thus an efficient nonradiative relaxation path from this state, which results in the broad spectrum. Again, this is due to vibronic coupling between the two states [109,173,174]. 

Another example of the role played by a nonradiative relaxation pathway is found in the photochemistry of octatetraene. Here, the fluorescence lifetime is found to decrease dramatically with increasing temperature [175]. This can be assigned to the opening up of an efficient nonradiative pathway back to the ground state [6]. In recent years, nonradiative relaxation pathways have been frequently implicated in organic photochemistry, and a number of articles published on this subject [4-8]. 

Figure 8.21 shows schematically a set of lx, 2s, 2p and 3s core orbitals of an atom lower down the periodic table. The absorption of an X-ray photon produces a vacancy in, say, the lx orbital to give A and a resulting photoelectron which is of no further interest. The figure then shows that subsequent relaxation of A may be by either of two processes. X-ray fluorescence (XRF) involves an elecfron dropping down from, say, fhe 2p orbifal fo fill fhe lx... 

In order to prevent this occurring a pulsed method of pumping is used with a repetition rate low enough to allow time for Tj — Sq relaxation. For CW operation either Tj must be sufflciently short or another dye has to be used for which T2 — Ti absorption does not overlap with the fluorescence. 

An alternative mechanism of excess energy release when electron relaxation occurs is through x-ray fluorescence. In fact, x-ray fluorescence favorably competes with Auger electron emission for atoms with large atomic numbers. Figure 16 shows a plot of the relative yields of these two processes as a function of atomic number for atoms with initial K level holes. The cross-over point between the two processes generally occurs at an atomic number of 30. Thus, aes has much greater sensitivity to low Z elements than x-ray fluorescence. 

The requited characteristics of dyes used as passive mode-locking agents and as active laser media differ in essential ways. For passive mode-locking dyes, short excited-state relaxation times ate needed dyes of this kind ate characterized by low fluorescence quantum efficiencies caused by the highly probable nonradiant processes. On the other hand, the polymethines to be appHed as active laser media ate supposed to have much higher quantum efficiencies, approximating a value of one (91). 

Quinacrine concentrates in the scolex of the parasite and causes the muscles needed for holding onto the intestinal wall to relax. The worms are stained yellow and pass from the body, still aUve. Quinacrine can intercalate with DNA and inhibit nucleic acid synthesis. It creates fluorescent bands in deoxyadenylate—deoxythmidylate-rich regions of DNA and has been used as a stain in the study of human genetics. 

Fast concentration and sample injection are considered with the use of a theory of vibrational relaxation. A possibility to reduce a detection limit for trinitrotoluene to 10 g/cnf in less than 1 min is shown. Such a detection limit can by obtained using selective ionization combined with ion drift spectrometry. The time of detection in this case is 1- 3 s. A detection technique based on fluorescent reinforcing polymers, when the target molecules strongly quench fluorescence, holds much promise for developing fast detectors. 

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