Transient phenomena decay

Creep deformation can be split into three separate parts. The first, transient creep, is a short lived phenomenon which gives a high initial rate of deformation but decays according to the expression ... 

We have carried out time-resolved EL measurements of polymer-based light-emitting devices. The transient EL spikes exhibit bi-exponential decay pattern. Temperature independence of the decay pattern allows us to rule out polarization due to polymer matrix relaxation as an EL governing process. According to our interpretation, the phenomenon of double light spikes under pulsed electrical... 

Formation and decay of transient intermediates constitute a phenomenon that is ubiquitous to enzyme catalysis. Time-resolved UV/visible optical spectra which document the absorbance spectral changes that accompany the conversion of chromophoric reactants to final products by way of transient intermediates have the potential for identification of these transient species. The RSSF experiment provides three-dimensional data sets consisting of optical absorbance as a function of both wavelength and time. Owing to the temporal nature of the acquired data, an RSSF experiment can be a powerful tool for sorting out the timing of intermediate formation and decay with respect to other events that take place during... 

Gerner and Schneider (1975) were the first to demonstrate that heat shock induces a transient state of heat resistance in mammalian cells. Subsequent research has demonstrated that cells or embryos given a sublethal heat shock sufficient to induce hsp synthesis exhibit tolerance to an otherwise lethal heat shock (Li and Laszlo 1985 Mirkes 1987 Johnston and Kucey 1988 Riabowol et al. 1988). This phenomenon has been termed acquired thermotolerance, and is defined for the purposes of this review as selftolerance. Data from numerous studies demonstrate that the acquisition of thermotolerance is attributable to the induction and expression of stress proteins (Landry et al. 1982 Welch and Mizzen 1988). Many investigations, including those of Landry et al. (1982), Li and Werb (1982), and Lavoie et al. (1993), have demonstrated that the kinetics of thermotolerance induction and decay are correlated with stress protein synthesis and degradation, respectively. 

An additional source of non-exponential emission decays is the transient effect that might appear at short times following excitation. This effect is frequently found in collisional quenching controlled by diffusion. In this situation, the quenching rate depends on the encounter probability between the fluorophore and the quencher, which is obviously also dependent on the diffusion coefficient and on quencher accessibility at early times, fluorophores that have quenchers located at short distances will react almost immediately , leaving behind just those that have to diffuse to encounter a quencher centre. The phenomenon is going to be perceived as a quenching rate that is time dependent at early times, and results in a faster decay component of the fluorophore emission [93]. 

While important, transient effects often pass unnoticed due to limited time resolution of the experimental apparatus or due to the small magnitude of the effect in fact, the phenomenon is more easily detected on slow diffusion processes in viscous media and long fluorescence lifetimes. The non-exponential intensity decay, resulting from a transient effect is described by Eq. (15.62), where a and b depend on diffusional parameters (diffusion coefficient and collision distance) and quencher concentration. 

Polymers are not homogeneous in a microscopic scale and a number of perturbed states for a dye molecule are expected. As a matter of fact, non-exponential decay of luminescence in polymer systems is a common phenomenon. For some reaction processes (e.g, excimer and exciplex formation), one tries to fit the decay curve to sums of two or three exponential terms, since this kind of functional form is predicted by kinetic models. Here one has to worry about the uniqueness of the fit and the reliability of the parameters. Other processes can not be analyzed in this way. Examples include transient effects in diffusion-controlled processes, energy transfer in rigid matrices, and processes which occur in a distribution of different environments, each with its own characteristic rate. This third example is quite common when solvent relaxation about polar excited states occurs on the same time scale as emission from those states. Careful measurement of time-resolved fluorescence spectra is an approach to this problem. These problems and many others are treated in detail in recent books (9,11), including various aspects of data analysis. 

The most useful approaches for obtaining information regarding the existence of intermediates and their lifetimes are fast reaction methods that mix enzyme and substrate within milliseconds, which permits the observation of single turnover events by various spectroscopic methods. Alternatively the reaction is rapidly quenched at known time intervals and its progress is analyzed chromatographically. In many cases in which an intermediate accumulates to the level of the enzyme concentration, such methods reveal the presence of burst kinetic that feature the rapid buildup of the intermediate in the transient phase followed by its slower rate of formation/decay in the steady state. The simplest kinetic scheme consistent with this phenomenon is given by... 

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