Total Quenching Rate Constants

The past few years have seen a maJcM- advance in quantitative measurements on noble gas atom reactions. Such studies divide into two major groups, dqxnding on whether the total rate of removal of the excited noble gas atom or the rate of a specific reaction channel is being measure. Up till now, the form technique has given the more reliable results in that it is possible to work under pseudo-first-order conditions with an excess of quenching agent, so that only relative excited atom concentrations are required. In contrast, measurement of the rates of specific channels requires in general the absolute rate of appearance of the [ oducts and the absolute concentration of excited atoms, both of which measurements are subject to systematic errors. 

At present, both types of measuranent are important aids to our understanding of the reactions of excited noble gas aunns, and experintental methods and results are discussed in this section. 

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In the present case, = 1.1x10 M- s i, a typical value for the reaction of aminoacid moieties with 1O2 (Michaeli Feitelson, 1994 Bisby et al., 1999). By comparison with the total quenching rate constant, kfi = 2.7x10 M- s-i, it can be concluded that almost 60% of the interaction with 1O2 is through physical quenching and about 40% of the reactive moieties of GA are oxidized by 1O2. 

Figure 48. Temperature dependence of total quenching rate constant for He + Ar. Solid curves represent measurements by Lin-dinger et al. MI dashed curves are calculated from optical potentials due to other authors.100 102...

Singlet oxygen total quenching rate constants (k +... 

TaUe 6 Total quenching rate constants At ( i i, Pi). (10 x rate coTtstants/cra molecule s quenching cross-sectUms/X given in parentheses)... 

Table 7 Total quenching rate constants. Part A Ar( Po,2). (lO " x rate constant/ cm molecule s" quenching cross-section/given in parentheses)...

Table 8 Total quenching rate constants Kr( i 2), Xe(Vj). (10 X Rate conr...

Discussion of Rate Data.— The measurements of the rates of specific quenching channels are now beginning to give some insight into the products of the quenching process, but it is less certain to what extent the channel determines the rate of reaction. Numerous models have been proposed to explain the measured total quenching rate constants, some based on the observed reaction channels " and others considering only the dynamics of the approach of the reactants. - As more information... 

Reaction Mechanisms in the Noble Gas Afleisfows.— The above survey of the electronic states of the noble gas molecules allows discussion of the kinetic data accumulated on the pure noble gas afterglows. These data comprise total quenching rate constants for excited atomic states, whidi are summarized in Table 3, and comparisons of the kinetics of atomic emissions with those of the first and second continua, from vdiich attempts have been made to elucidate the mechanisms for the formation of the emitting molecular states. 

The following total quenching rate constants k (in 10" cm -molecule" s" ) and the branching fractions T (ratio of the rate constant for the formation of an individual species to the total quenching rate constant) were obtained [8,11] ... 

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