Annihilation rates determination

The positron lifetimes for different defects in MgO are calculated using the insulator model of Puska and co-workers. In this model, the annihilation rates are determined by the positron density overlapping with the enhanced electron density that is proportional to the atomic polarizability of MgO [8, 9]. Based on comparison between experimental and calculated values [5, 6], the positron lifetime of the embedded Au nanoparticle layer, 0.41 ns, suggests that positrons are predominantly trapped in clusters consisting of... 

The exciton-exciton annihilation rate constant for singlet excitons in solid films of poly(n-propylmethylsilane) was determined by measuring the fluorescent intensity from the films as a function of incident-light intensity in two types of experiments one in which the excitons were created by single-photon transitions and one in which the excitons were created by two-photon transitions. The rate constant is cm ls. 

The Sj-Sj annihilation rate constant was determined in the following manner. First, the concentration of the excimer was obtained by dividing the observed absorbance by its extinction coefficient and by the effective cell length where the Sj<-Sp absorbance at 266nm was 1. In addition, in pure liquid benzene as well as in solution, there exists rapid equilibrium between monomer and excimer, of which time constants of association and dissociation are in the order of a few ps °. Hence, the sum of the monomer and excimer concentrations obtained by the equilibrium constant at each temperature was used as the concentration of the excited singlet species for the analysis. Although this assumption may affect to some extent the accuracy of the obtained rate constant, the error of this estimation would not depend upon the temperature. 

To date, the self-annihilation rate constant for NCI (a) has only been determined using photolysis techniques. Henshaw et were able to... 

The constants of the equation were obtained from simulations and the process applies only to generation of singlet excited states at diffusion-limited annihilation rates. Nonetheless, the expression provided an experimental approach for determining efficiencies of the production of emitting excited states in annihilation reactions. Simulations for systems that react via triplet formation and subsequent triplet-triplet annihilation were also developed [40b] and they illustrated that the two mechanisms can be distinguished by analysis of intensity-time profiles. 

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... 

The l S,-2 Si interval in Ps is attractive for a precision measurement because its natural line width is determined solely by the annihilation rate of the triplet states and is small compared to the 1 S 2S interval, so that the fractional... 

This can be accomplished by setting up appropriate kinetic equations arid subsequent integration of the resulting differential equations, from which the population of the various states in which the positrons exist o-Ps and PsM can be found as a function of time. From these values and the positron annihilation constants for these states, an equation for the time dependent two photon annihilation rate can be obtained, which in turn allows the determination of the chemical reaction rate constants by utilizing sophisticated nuclear chemical lifetime measurement techniques. 

In order to obtain rate constants for the reaction of Ps (or positrons) with substrate molecule or to follow changes in the reactivity of a certain medium towards Ps the two photon annihilation rate (see above) has to be determined. This is accomplished by positron lifetime conventional fast-slow y y coincidence methods. 22... 

In order to determine the effect of the microstructure of the solution on the beta counting efficiency in toluene -Triton mixtures in a first series of experiments the positron annihilation parameters were determined in toluene - Triton mixtures containing various amounts of water. As can be seen from Eigs. 11 and 12, where the Parameter l2 which is correlated to the formation probability of thermal positronium, is plotted as a function of Triton (in the presence of 0 and 2 % water) concentration, increasing amounts of Triton reduce I2 to a semi-plateau value, while X2, the annihilation rate of the thermal positronium, changes only slightly. A more detailed plot of I2 at lower Triton concentration reveals that I2 remains constant up to 20 mM or 10 mM Triton in solutions containing 0 or 2 % water, respectively. 

The diffusion kinetics were studied, at 220 to 270K, by analyzing the photo-induced dissociation of an etchant-generated D-C complex. Under suitably strong illumination, the annihilation rate of the complex was proportional to the P density. This indicated that the rate-determining step was the diffusion of D to P atoms. By invoking diffusion-controlled reaction theory, it was deduced that the diffusion was described by ... 

The transformation of the annihilation rate PDF that is, a(A) to the corresponding radius PDF for the free-volume regions, the sites of o-Ps annihilation is conveniently determined using Newton s method. Therefore, the radius (R) PDF is... 

When a positron is emitted from a source, and penetrates into a solid, it quickly loses its kinetic energy to thermal energy. The thermalised positron moves around in the solid by diffusion and finally annihilates with one of the electrons in its surroundings. All of the energy from the electron-positron annihilation is converted into two annihilation y-rays, which can be detected. The annihilation rate of a positron is determined by the local electron density in the locale of the positron. Thus, positrons... 

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