Annihilation mechanism

Energy-deficient radical-ion reactions generally exhibit low quantum yields due to the rather circumstantial triplet-triplet annihilation mechanism 150>. 

Another pumping path is also possible by T-T annihilation mechanism hv... 

The line can excite TPPPd but not TBPCd. The pumping is performed by T-T annihilation mechanism as follows (26) ... 

If the S2 fluorescence is mainly produced by the triplet-triplet annihilation (mechanism [A]), the decay profile must depend upon the decay rate constant (k ) of the first excited triplet state, and... 

An intriguing complication arises when a series, say, of uphill steps is followed by an equal number of downhill steps, forming a one-dimensional wire on the surface. Below roughening, the thermal flattening of the wire proceeds through the top step annihilation mechanism -. By that, the amplitude of the wire is reduced by one lattice... 

In a suggested alternative mechanism, lower energy triplets produced by the cation-anion annihilation produce fluorescent singlets by triplet-triplet annihilation.73 This possible pathway cannot be overlooked on two accounts. First, it is known from phosphorescence emission that triplets are produced in these systems13,15,60 and second, the double potential step data analyzed by the Feldberg method seems to be in agreement with such a double annihilation mechanism.65 If triplets are initially produced in high local concentrations by the redox reaction in... 

Here p is a defect creation rate per unit time and volume, called also dose rate, f r) is their initial distribution function over relative distances, normalised according to f f(r)dr= 1, o(r) the AB pair recombination rate. For the annihilation mechanism o(r) = cro0(ro - r), 0 is the Heaviside step-function (Section 3.1). 

Figure 7.15 shows the joint correlation function of similar defects in the region of concentration saturation as calculated from the results of the simulation. We see that the fraction of the close Frenkel defects (of the type of dimer F2-centres) exceeds by approximately threefold the value expected in the Poisson distribution, which agrees well with the analytical theory presented in Section 7.1 for the annihilation mechanism (see also [31, 111]) and with actual experiments for alkali halide crystals [13]. 

With the advent of rapid-scan and high-frequency pulse methods, more direct approaches for evaluating annihilation mechanisms and dynamics have been developed. Early work of van Duyne, using triple potential steps with very short step times, allowed estimation of the annihilation rate constant for DPA anion and cation radicals [29]. More recently, Wightman and coworkers have used multicycle generation of ECL at microelectrodes to determine annihilation rate constants and ECL efficiencies [41, 42]. Figure 7 shows the normalized ECL intensity from DPA at a 1-pm Pt disk as a function of time (t/tf) at different oscillation frequencies. The intensity increases rapidly after the potential is switched, and then decays as the reactants are depleted. As the oscillation frequency is increased, the annihilation occurs closer to the electrode surface, the intensity-time profile broadens and... 

A triplet exciton annihilation mechanism has been proposed for charge generation in molecularly doped polymers initiated with very fast, high-intensity, excitation [25b,c]. In this mechanism it is proposed that both charge separation, associated with the Onsager model, and the fusion process are very strongly field-dependent. This model has not been invoked in recent years. 

Thus, the annihilation mechanism for each of the lifetime components of the spectra are of very different nature. The annihilation mechanism which increases with burn-off is different depending on the activating agent used. For the ACFs obtained from CO2 activation (CFC) the main annihilation mechanism corresponds to the annihilation of free positrons with surface electrons (L) in contrast to the case of the ACFs obtained from steam (CFS) where the mechanism that increases with burn-off is the o-Ps annihilation (I2) (Table 4). 

Good correlations have been found between this new technique and others typically used to characterize porous materials (i e., SAXS and gas adsorption). The results obtained show a direct relationship between positrons lifetime and pore size. An increase of pore size produces an increase in the positron lifetime Additionally, PALS is sensitive to structural changes produced during CO2 and steam activation. Thus, the annihilation mechanism which is favoured with increasing the burn-off is different depending on the activating agent... 

Non-exponential phosphorescence decay is frequently observed for various aromatic chromophores molecularly dispersed in polymer matrices. Various possible mechanisms for non-exponential decay are reviewed, and a dynamic quenching mechanism by polymer matrices including the effect of a time-dependent transient term in the rate coefficient is discussed in some detail. The biphotonic triplet-triplet annihilation mechanism is also introduced for the non-exponential decay under high-intensity and/or repeated laser irradiation. 

The occurrence of non-exponential decays in poly(vinyl alcohol) where there is no possibility of triplet energy migration is an additional proof of the absence of the T-T annihilation mechanism in the phosphorescence decay of benzophenone in polymer matrices. 

The decay rate of triplet concentration in the presence of a T-T annihilation mechanism is given by... 

The non-exponential decay of benzophenone phosphorescence in PMMA at room temperature was also observed by Fraser et al. (36), under the condition of repeated irradiation of a nitrogen laser pulse, and they proposed a triplet-triplet annihilation mechanism in which the polymer matrix itself participated as an energy acceptor from benzophenone triplet and a medium for the triplet energy migration. 

Radiative and Non-radiative Processes.—The variation of the ratio of phosphorescence to fluorescence for CeH5, C De, CeH6CH8, and - 4( )2 in methylcyclohexane at 77 can be accounted for by an increase in the importance of triplet-triplet annihilation at high solute concentration. The detection of delayed fluorescence in these systems supports the triplet-triplet annihilation mechanism.184 This conclusion is different from that reached in earlier reports,186 when it was suggested that an increase in concentration of the aromatic affected the efficiency of intersystem crossing (Sj, -> T ). 

Since the positron is the antiparticle of the electron, an encounter between them can lead to the subsequent annihilation of both particles. Their combined rest mass energy then appears as electromagnetic radiation. Annihilation can occur via several mechanisms direct transformation into one, two, or three photons or the formation of an intermediate, hydrogen-like bound state between the positron and the electron, known as a positronium (Ps). The extent to which each annihilation mechanism contributes depends on the kinetic energy of the positron-electron pair. 

No evidence for the existence of a lifetime component characteristic of the crystal nanochannels was found, indicating that the second annihilation mechanism is... 

There are two reaction processes to generate ECL. The first detailed study about ECL was focused on annihilation mechanism induced by electron-transfer reactions between an oxidized and a reduced species. 9,10-Diphenylanthracene (DPA) was used as the emitting molecule for the annihilation mechanism in the early stage [5, 6]. A proposed mechanism is shown below. 

El-Sayed et al. interpreted the non-exponential decays of naf thalene and triphenylene in PMMA at room temperature in terms of a triplet-triplet annihilatiexcitation intensity and the observation of delayed fluorescence. El-Sayed et al. suggested that a non-exponential decay should be obsm%d for molecules with first-order lifetimes of the order of several seconds. However, coronene with the triplet lifetime of 8.S s gave an exponential decay. These authors also suggested a rather high concentration of chromophore at the excited triplet state on the basis of the diffusion-controlled triplet-triplet annihilation mechanism. Later, Jassim et al. 

The steady-state dynamics is assumed to be governed by a Kawasaki-type particle-vacancy NN pair-exchange mechanism inside the system combined with a Glauber-type particle creation/annihilation mechanism at the two edges A and B. Hence, neither the particle number nor the total energy are conserved quantities. This implementation corresponds to a canonical ensemble inside the lattice and a grand canonical ensemble at the edges. The total density is hence a dependent variable which has to be calculated. The dynamical processes are subject to the conventional Monte Carlo Metropolis criterion. ... 

Figuic 9. Formation mechanisms of APB tubes, (a) The Vidoz-Brown non-aligned jog mechanism, (b) The cross-slip and annihilation mechanism... 

As with all chemically amplified resists, a major concern is, however, the latent image stabflity and the susceptibility to environmental conditions. With t-BOC deprotecdon systems, the influence of airborne nudeophilic contaminants has been recently demonstrated (23) the observadon of surface residues in a number of such materials (23, 24) may be traced back to the presence of ppb amounts of volatile bases. In the case of the acetal systems (19-21), the influence of trace bases is less pronounced, as even amine hydrochlorides are sdll sufficiendy addic to have some catatydc activity. Linewidth dianges with the interval between exposure and post exposure bake have been observed for both the t-BOC and the acetal systems. In the case of the t-BOC tystems, long intervals (several hoius) between exposure and post-e]q)osure bake will lead to a decrease of apparent sensidvity, which manifests itself as a linewidth inCTease, or, in extreme cases, as faUure to open the imaged areas. These effects are normally due to contaminadon by base traces, or, in cases where the presence of even ppb amounts of bases can be excluded, may be assumed to be the result of the same, unspecified chain terminadon (add annihilation) mechanism which is responsible for the containment of the calalytic reacdon to the immediate vicinity of the imaged resist. 

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