Electron capture rate constant

Sauer, M. C. and Schmidt, K. H., Electron capture rate constants for solutes in alkane liquids measured by transient photoconductivity, Radiat. Phys. Chem., 43, 413,1994. 

In order to later estimate thermal electron capture rate constants, Van Doren et have calculated the polarizabilities of the SF C1... 

Analytes that are not suitable candidates for electron capture can often be made so by suitable derivatization. Perfluoracyl, pentafluorobenzyl, pentafluorobenzoyl and nitrobenzyl derivatives are often used not only to increase the electron capture rate constants but also to enhance chromatographic properties. However, in contrast to electron capture gas chromatography, it is not sufficient that electron capture be facile in addition, ions characteristic of the analyte must be formed since ions characteristic only of the derivatizing agent leaves the identity of the analyte in question. This is illustrated by the data in Table 4 for derivatization of phenols, where it can be seen that the perfluoroacyl derivatives yield ions characteristic only of the derivatizing agent, whereas the pentafluorobenzyl and pentafluorobenzoyl derivatives yield ions characteristic of the analyte. Also... 

In case of electron scavenging (and no Ps lifetime quenching, as is true for both Cl" and Tl+), no other positron states are present than free e+ and Ps then, the intensities from PALS and from DB are the same. The p-Ps and o-Ps intensities are expected to decrease so that the fwhm of the DB spectra should increase with solute concentration (the narrow components are suppressed). The variations of fwhm with C can be completely calculated, knowing the intensities Ij from PALS and the Tj previously established for a given solvent. This is illustrated by the solid line in Figure 4 for Tl+ this ion, as expected from its high solvated electron scavenging rate constant, is thus shown to suppress Ps formation by electron capture. 

We saw that the bulk recombination rate was given by eqn. (351) where Cn and Cp are the trap capture rate constants (s 1) and nlr and ptr are defined above. At the surface, the relevant parameter of interest is the surface recombination rate, S, the number of recombining electron-hole pairs per unit area per second, which can be expressed [2] by... 

Table 5. Some electron capture agents and their capture rate constants.

For Eq. (3.10) and Eq. (3.12) the bimolecular rate constants can be replaced with pseudo unimolecular rate constants within the limits of either fractional electron capture or constant positive ion concentration. All the above reactions take place on a time scale that is fast relative to the time required for transport through the detector. Under steady state conditions the electron capture coefficient K (see Eq. 3.7) is given by... 

Quasiequilibrium statistical theory was applied to the negative ion mass spectra of diphenylisoxazoles. Electron capture by the isoxazole leads to molecular ions having excited vibrations of the ring and of bonds attached to it. The dissociation rate constants were also calculated (77MI41615, 75MI416U). 

Figure 4, Structure of a series of alkyl bromides and the rate constants (1 O " cm s ) for the IM reaction of each compound with the chloride ion, determined by the photodetachment-modulated electron capture detector (PDM-ECD) in 10% argon-inmethane buffer gas at atmospheric pressure and 125 °C. ...

For use in geochronology, the decay constant of a radioactive nuclide must be constant and must be accurately known. For a-decay and most (3-decays, the decay constant does not depend on the chemical environment, temperature, or pressure. However, for one mode of 3-decay, the electron capture (capture of K-shell electrons), the decay "constant" may vary slightly from compound to compound, or with temperature and pressure. This is because the K-shell (the innermost shell) electrons may be affected by the local chemical environment, leading to variation in the rate of electron capture into the nucleus. The effect is typically small. For example, for Be, which has a small number of electrons and hence the K-shell is easily affected by chemical environments, Huh (1999) showed that the decay constant may vary by about 1.5% relative (Figure l-4b). Among decay systems with geochronological applications, the branch decay constant of °K to °Ar may vary very slightly (<1% relative). 

Shortly after the discovery of the hydrated electron. Hart and Boag [7] developed the method of pulse radiolysis, which enabled them to make the first direct observation of this species by optical spectroscopy. In the 1960s, pulse radiolysis facilities became quite widely available and attention was focussed on the measurement of the rate constants of reactions that were expected to take place in the spurs. Armed with this information, Schwarz [8] reported in 1969 the first detailed spur-diffusion model for water to make the link between the yields of the products in reaction (7) at ca. 10 sec and those present initially in the spurs at ca. 10 sec. This time scale was then only partially accessible experimentally, down to ca. 10 ° sec, by using high concentrations of scavengers (up to ca. 1 mol dm ) to capture the radicals in the spurs. From then on, advancements were made in the time resolution of pulse radiolysis equipment from microseconds (10 sec) to picoseconds (10 sec), which permitted spur processes to be measured by direct observation. Simultaneously, the increase in computational power has enabled more sophisticated models of the radiation chemistry of water to be developed and tested against the experimental data. 

Figure 11. Thermal rate constants for capture of electronic ground state O2 by an ion (SACM calculation [33] with channels generating from rotational states J = 0,1,2, accounting for the open-shell character of O2 left figure positive ion right figure negative ion).

It has been suggested in the literature that the a-amino radical is the species that initiates polymerization [210], This view is supported by our observation that, in spite of the relatively high quenching rate constant of Eosin triplet by triphenylamine (Table 5), the system Eosin-triphenylamine does not sensitize the photopolymerization of multifunctional acrylates. Thus, it is necessary that the amine contains a hydrogen at the a-carbon to be released as a proton after oxidation of the amine by the dye triplet. This deprotonation prevents the back electron transfer and forms a carbon radical that is sufficiently long-lived to be captured by the monomer. 

Pulse radiolysis studies on unsymmetrical, chemically linked organic systems have shown the expected fall off in rate constant as AG becomes more favorable.81 In these experiments, advantage is taken of the fact that capture of electrons produced by pulse radiolysis is relatively indiscriminate and in some of the pulse events an electron is captured by the component in the dimeric systems which is the weaker oxidant. Following such an event, the experimental observation made is of the system relaxing by intramolecular electron transfer to the stable redox configuration, as shown for (I)->(2) where A is one of a series of polyaromatic or quinone electron acceptors.81... 

The electron e generated by irradiation via ionization of the molecules of the medium S is thermalized (et ) and then is either captured by a trap, T (rate constant k ) to yield et or reacts with an acceptor, B (rate constant k2). Scheme (24) leads to a linear dependence of the value of G"1 on the acceptor concentration... 

The rate constants of reactions of hydrated electrons with some accep-tors-anions substantially exceed the diffusion rate constants calculated with the help of the Debye equation [Chap. 2, eqn. (45)l(see Chap. 2, Sect. 4). This excess is usually attributed to the capture of electrons by acceptors via tunneling at distances exceeding the sum of the reagents [28,89,111,1201- In this case, the tunneling distance can be estimated from experimental rate constants for reactions of eaq with acceptors [109] by means of the expression... 

Effective rate constants VJt of the light-induced decay of trapped electrons and the radii, i T, of the capture of et by acceptors calculated from the kinetics of etr photobleaching by means of eqn. (28)... 

Two suggestions have been advanced as to why the rate constant does not vanish at low n. First, Beterov et al.91 have suggested that before the electron is captured by the SF6 n increasing collisions with SF6 lower the binding energy to the point that coulomb trapping does not eliminate SF6 production. In their... 

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