Kinetic intensity time profiles

Thus, the question of central concern raised in our contribution has been the macroscopic formulation of EET and its relation to the experimental observable of excimer fluorescence in a time-resolved experiment. EET has been discussed, hers, as a dispersive, i.e., time-depen-dent process in deterministic monomer-excimer models which had been the subject of a detailed kinetic analysis in recent work (3 8, 4.S.). With the use of rate function k(t) (Equation 4) it is natural to yield typical non-exponential intensity-time profiles, either in form of an asymptotic approach (Equations 5,6), or in closed form analytical solutions (Equations 7,8). The physios emer-... 

In the absence of analyte, many CL systems show a low emission background level. Hence, in flow systems, as the CL intensity is proportional to the analyte concentration, the emission appears as a sharp peak superimposed on a low constant blank signal, which is measured when the mixture of analyte and CL reagents passes through the detector cell. Because only a small portion of CL emission is measured from this time profile, nonlinear calibration curves may be obtained for reactions with complex kinetics [1],... 

The first detailed investigation of the reaction kinetics was reported in 1984 (68). The reaction of bis(pentachlorophenyl) oxalate [1173-75-7] (PCPO) and hydrogen peroxide cataly2ed by sodium saUcylate in chlorobenzene produced chemiluminescence from diphenylamine (DPA) as a simple time—intensity profile from which a chemiluminescence decay rate constant could be determined. These studies demonstrated a first-order dependence for both PCPO and hydrogen peroxide and a zero-order dependence on the fluorescer in accord with an earher study (9). Furthermore, the chemiluminescence quantum efficiencies Qc) are dependent on the ease of oxidation of the fluorescer, an unstable, short-hved intermediate (r = 0.5 /is) serves as the chemical activator, and such a short-hved species "is not consistent with attempts to identify a relatively stable dioxetane as the intermediate" (68). 

The bimodal profile observed at low catalyst concentration has been explained by a combination of two light generating reactive intermediates in equihbrium with a third dark reaction intermediate which serves as a way station or delay in the chemiexcitation processes. Possible candidates for the three intermediates include those shown as "pooled intermediates". At high catalyst concentration or in imidazole-buffered aqueous-based solvent, the series of intermediates rapidly attain equihbrium and behave kineticaHy as a single kinetic entity, ie, as pooled intermediates (71). Under these latter conditions, the time—intensity profile (Fig. 2) displays the single maximum as a biexponential rise and fall of the intensity which is readily modeled as a typical irreversible, consecutive, unimolecular process ... 

The kinetic expression for the time—intensity profile (I vs /) for this model is given by the following... 

The typical profile of a CL transient signal (a plot of CL intensity vs. time) is a kinetic response curve that corresponds to a first-order sequence of two consecutive steps, namely (1) generation of the light-emitting product by mixing of the chemical ingredients (the substrate and oxidant), and (2) formation of the end product (Fig. 2). The rate at which each step takes place depends on the formation... 

The objective of the present work was to determine the influence of the light intensity on the polymerization kinetics and on the temperature profile of acrylate and vinyl ether monomers exposed to UV radiation as thin films, as well as the effect of the sample initial temperature on the polymerization rate and final degree of cure. For this purpose, a new method has been developed, based on real-time infrared (RTIR) spectroscopy 14, which permits to monitor in-situ the temperature of thin films undergoing high-speed photopolymerization, without introducing any additive in the UV-curable formulation 15. This technique proved particularly well suited to addressing the issue of thermal runaway which was recently considered to occur in laser-induced polymerization of divinyl ethers 13>16. 

An important quantity that can be deduced from the reaction profile is the rate of the cross-linking polymerization (Rp), i.e., the number of double bonds polymerized or of cross-links formed per second. Rp values were determined from the maximum slope of the kinetic curves (usually reached for conversion degrees between 20 and 40%). Table I summarizes the Rp values for the two photoresists tested under various conditions, namely conventional UV and continuous or pulsed laser irradiation at different light intensities. According to these kinetic data, Rp increases almost as fast as the light-intensity the ratio Io/Rp which is directly related to the product of the light-intensity and the required exposure time was found to vary only in the range 10-8 to... 

Figure 7.32 Kinetics of luminescence of pyrene following laser flash excitation. L, laser pulse profile M, monomer emission, E, excimer emission rise and decay. Horizontal axis, time in ns vertical axis, light intensity in arbitrary units. The three kinetic curves are normalized to a common maximum...

Axial dispersion can affect measurements of decay and growth rates of transients of interest. In Figure 5 is sketched the concentration of a transient, initially formed as a square wave by a light pulse of uniform intensity from — L < x < L and zero elsewhere. As shown in Figure 6, at later times the profile becomes smoothed by diffusion. As the purge flow pushes reactive species past the pinhole at x = 0, the spatial dependence of the concentration becomes a time-varying concentration that will contribute to any time variation caused by kinetics. 

Phenyltrimethyldisilene (15) and (E)- and (Z)-l,2-dimethyl-l,2-diphenyldisilene (16) were also observed as transient absorption spectra by laser flash photolysis of the precursors in methylcyclohexanes28. The absorption band at 380 nm, assigned to the disilene 15, reached maximum intensity at ca 10 ns after the excitation and then started to decrease. The half-life assigned to 15 was 700 ns. The logarithm of the decay profile of the transient absorption at 380 nm versus time shows a very good linear relationship, indicating that the decay of the transient absorption fits first-order kinetics. This result shows that intramolecular isomerization or proton abstraction from the solvent is the origin for the decay of the disilene 15, which survives in solution only for several nanoseconds. 

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