Decay times, molecules

However, it has been pointed out 13 16> for large organic molecules ( statistical limit case) that the decay times and quantum yields can legitimately be handled by the Fermi golden rule ... 

Stimulation of the motoneuron releases acetylcholine onto the muscle endplate and results in contraction of the muscle fiber. Contraction and associated electrical events can be produced by intra-arterial injection of ACh close to the muscle. Since skeletal muscle does not possess inherent myogenic tone, the tone of apparently resting muscle is maintained by spontaneous and intermittent release of ACh. The consequences of spontaneous release at the motor endplate of skeletal muscle are small depolarizations from the quantized release of ACh, termed miniature endplate potentials (MEPPs) [15] (seeCh. 10). Decay times for the MEPPs range between l and 2 ms, a duration similar to the mean channel open time seen with ACh stimulation of individual receptor molecules. Stimulation of the motoneuron results in the release of several hundred quanta of ACh. The summation of MEPPs gives rise to a postsynaptic excitatory potential (PSEP),... 

The fluorescence decay time, calculated for the 400 nm emission of TIN in PMMA films, decreases from 1.3 0.2 ns to 0.20 0.02 ns as the concentration of TIN is increased from 0.07 mole% to 1.1 mole% respectively. This is further evidence that the TIN molecules are involved in a concentration-dependent, self-quenching... 

We simulated [38] the orientation TCF for sub-ensembles of molecules that have different OH stretch frequencies at t 0. We found that within 100 fs there was an initial drop that was frequency-dependent, with a larger amplitude of this drop for molecules on the blue side of the line. For times longer than about 1 ps the decay times for all frequencies were the same. We argued that since molecules on the red side of the line have stronger H bonds, they are less free to rotate than molecules on the blue side, leading to a smaller initial decay. For times... 

Triplet-triplet annihilation In concentrated solutions, a collision between two molecules in the Ti state can provide enough energy to allow one of them to return to the Si state. Such a triplet-triplet annihilation thus leads to a delayed fluorescence emission (also called delayed fluorescence of P-type because it was observed for the first time with pyrene). The decay time constant of the delayed fluorescence process is half the lifetime of the triplet state in dilute solution, and the intensity has a characteristic quadratic dependence with excitation light intensity. 

The fluorescence decay time is one of the most important characteristics of a fluorescent molecule because it defines the time window of observation of dynamic phenomena. As illustrated in Figure 3.2, no accurate information on the rate of phenomena occurring at time-scales shorter than about t/100 ( private life of the molecule) or longer than about 10t ( death of the molecule) can be obtained, whereas at intermediate times ( public life of the molecule) the time evolution of phenomena can be followed. It is interesting to note that a similar situation is found in the use of radioisotopes for dating the period (i.e. the time constant of the exponential radioactive decay) must be of the same order of magnitude as the age of the object to be dated (Figure 3.2). 

The surface-state model, in which the luminescent recombination occurs via surface states, was proposed to explain certain properties of the PL from PS, for example long decay times or sensitivity of the PL on chemical environment. In the frame of this model the long decay times are a consequence of trapping of free carriers in localized states a few hundred meV below the bandgap of the confined crystallite. The sensitivity of the PL to the chemical environment is interpreted as formation of a trap or change of a trap level by a molecule bonding to the surface of a PS crystallite. The surface-state model suffers from the fact that most known traps, e.g. the Pb center, quench the PL [Me9], while the kinds of surface state proposed to cause the PL could not be identified. 

The Dp and Dq are the diffusion coefficients of probe and quencher, respectively, N is the number molecules per millimole, andp is a factor that is related to the probability of each collision causing quenching and to the radius of interaction of probe and quencher. A more detailed treatment of fluorescence quenching including multiexponential intensity decays and static quenching is given elsewhere/635 There are many known collisional quenchers (analytes) which alter the fluorescence intensity and decay time. These include O2/19 2( 29 64 66) halides,(67 69) chlorinated hydrocarbons/705 iodide/715 bromate/725 xenon/735 acrylamide/745 succinimide/755 sulfur dioxide/765 and halothane/775 to name a few. 

As shown by Tagawa et al. [74], the alkane excited molecules have a broad absorption band in the visible region with maxima increasing from —430 to —680 nm between C5 and C20 for the -alkanes. This spectrum strongly overlaps with the absorption spectrum of the radical cations with low carbon atom number alkanes the two maxima practically coincide. With increasing carbon atom number, the red shift in the radical cation absorbance is stronger than in the Si molecule absorbance [47,49,84-86]. The decay of the excited molecule absorbance was composed of two components with 0.1- and 1.0-nsec decay times in cyclohexane, and 0.17 and 2.7 nsec in perdeuterocyclohexane [47]. The nature of the faster-decaying component is as yet unclear. 

This effective dye relaxation time rp is the spontaneous fluorescence decay time shortened by stimulated emission which is more severe the higher the excitation and therefore the higher the population density w j. The dependence of fluorescence decay time on excitation intensity was shown in 34 35>. Thus, fluorescence decay times measured with high intensity laser excitation 3e>37> are often not the true molecular constants of the spontaneous emission rate which can only be measured under low excitation conditions. At the short time scale of modelocking the reorientation of the solvent cage after absorption has occurred plays a certain role 8 > as well as the rotational reorientation of the dye molecules 3M°)... 

The authors believe that the decreases in decay times are associated primarily with changes in quantum yield. This may be inferred from the fact that both the emission intensities and lifetimes are falling off at about the same rate with temperature. One thus concludes that the luminescence of sulfuric acid solutions of terbium sulfate is subjected to much greater temperature quenching than the luminescence in aqueous solution of the same salt. The increasing probability of radiationless transitions is undoubtedly connected in some manner with greater interaction of the radiating ion with the solvent molecules. 

Solvation in water was extensively studied and processes on different timescales were described ranging from 30 fs to several ps [8]. Due to our experimental resolution the shortest decay time we measure contains various superimposed contributions from the ultrafast processes presumably the inertial response of water and initial librational motions of molecules in the first solvation layer. 

The simplest case of reaction kinetics occurs when the excitation and emission occur in the same atom, molecule, or luminescence center. The recombination can then be treated as a first-order unimolecular reaction. The decay time is independent of the number of other similarly excited atoms or molecules. 

Fig. 4. Experimental radiative decay times and fluorescence quantum yields for aromatic molecules in inert solvents. Data from (a) I. B. Berlman 202 (b) Strickler and Berg.28...

Although the general theory outlined provides a satisfying unified interpretation of the many relaxation processes mentioned, at present reliable numerical predictions are not possible. This must be considered the most serious technical limitation of the analysis we have reviewed. Because of the technical difficulties encountered in the a priori calculation of matrix elements, densities of states, etc., it is tempting to reverse the analysis to obtain information about the relevant intramolecular matrix elements, densities of states, etc., from line shape data and the several luminescence decay times. For example, it seems likely that the complex spectrum of a molecule such as NOa could be analyzed in this fashion, and thereby provide information not now available from any other source. 

The excited molecule is a unique surface probe. The decay time provides a clock which can be used to study such dynamics. Since both singlets and triplets are potential probes, the time scale extends from seconds to picoseconds. In addition, the nature of any photo-... 

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