Primary yield

Scavenging experiments in hydrocarbon liquids (Rzad et al, 1970 Kimura and Fueki, 1970) tend to give low observed ionization yield, although the primary yield may be greater. The situation is similar for free-ion yield measurement under a relatively large external field. Both processes require large extrapolations to obtain the W value. 

First, we will review the stationary primary yield of the hydrated electron at neutral pH for low-LET radiation at a small dose. The primary species are eh, H30+, H, OH, H2, and H2Or Material balance gives... 

Since charge conservation requires g(eh) = g(H30+), the latter yield will not be considered further. The chemical measurement of g(eh) uses Eq. (6.2) and the measurements of primary yields of H, H2, OH, and H202 in a suitable system. Various systems may be used for this purpose (see Draganic and Draganic , 1971). For example, in methanol solution radiolysis, H2 is produced by the reaction H + CH3OH—H2 + CH2OH. Therefore, in this system, G(H2) = g(H2) + g(H). If, in addition, there is excess oxygen, the H atoms would be removed by the reaction H + 02 H02. Therefore, from these two measurements, both g(H) and g(H2) may be obtained. 

The primary yield of H,02 may be obtained by measuring the H202 yield in a system containing excess oxygen. In this case, hydrated electrons and H atoms are converted into O,- and H02, respectively, with an equilibrium between these species ... 

In this manner, one obtains g(eh) = 2.7 at neutral pH. In the past, considerable controversy existed about these primary yields in view of work at different laboratories using slightly different techniques, under somewhat different conditions, and so forth (see Farhataziz et al, 1966). Table 6.1 illustrates the results and possible reconciliation. The controversies have been basically resolved, and most workers now agree that g(eh) = g(OH) = 2.7 0.1, g(H) = 0.56 0.05,... 

Adapted from Farhataziz et al. (1966). Note that these are observed yields from which the primary yields are calculated and referred to in the text. 

Farhataziz et al. (1974a, b) studied the effect of pressure on eam and found that as the pressure is increased from 9 bar to 6.7 Kbar at 23° (1) the primary yield of e decreases from 3.2 to 2.0 (2) hv increases from 0.67 to 0.91 eV (3) the half-width of the absorption spectrum on the high-energy side increases by 35% and (4) the extinction coefficient decreases by 19%, which is similar to eh. The pressure effects are consistent with the large volume of ean (98 ml/M), whereas the reduction in the observed primary yield at 0.1 ps is attributable to the reaction eam + NH4+. Some of the properties of eam have been discussed by several authors in Solvated Electron (Hart, 1965). 

In the Simple model, taking the primary yield as px and the secondary yield as piz, the primary abundance is... 

In principle, one should be able to calculate k8Jk8b from the observed quantum yields. In view of uncertainties in the NO quantum yield and the primary yield of O, however, it is not possible at the present time to obtain a reliable estimate for this ratio. 

The numbers in reaction (6) are the so-called primary yields (expressed here in units of 10 mol J ) of the radical and molecular products that are present when the spur processes are complete at ca. 10 sec. The original units for these yields, which are still in use, are molecules (100 eV) and the relationship between the two sets is 1 mol J =9.65x10 molecules (100 eV) Thus, reaction (6) may also be written as reaction (7) ... 

In this chapter, the various radiation chemical yields, known as G-values, are defined as follows g(X) is the yield of the species in reactions (6) and (7) G°(X) is the yield of the initial products of water radiolysis at the end of the physicochemical stage and G(X) is an experimentally measured yield. In some publications Gx, rather than g(X), is used to represent the primary yield of the species X. The yields in reactions (6) and (7) are numerically very similar, but one must be sure of the units that are being used to express them. In the older literature, it was common practice to quote G-values without units when the units were in fact molecules (100 eV) ... 

By comparison with G e, relatively few independent measurements of G( OH) have been made. In contrast to, only the relative change in G( OH) with time has been reliably measured by pulse radiolysis [51]. In practice, absolute values of G( OH) have been obtained from scavenger studies or by material balance (reaction (7)). Fig. 7 shows data for aerated solutions of formate ion [52] and hexacyanoferrate(II) [53] taken from Fig. 1 of Ref. 54. The data for formic acid, which were included by LaVerne and Pimblott [54], have been omitted here because they were obtained at low pH where the primary yields are different (see Section 3.4). The solid line shows the best fit obtained using Eqs. (16) and (17) and the broken line is the best fit when the term u[5]/2 is omitted from Eq. (17). The respective sets of parameters are a = 1.64 and 1.69 nsec, g( OH) = 2.53 and 2.50 molecules (100 eV) and G°( OH) = 4.48 and 4.86 molecules (100 eV) These values differ significantly from those obtained by LaVerne and Pimblott [54], which were a = 0.258 nsec, g(" OH) = 2.66 molecules (100 eV) and G°( OH) = 5.50 molecules (100 eV) The reason for the difference is that LaVerne and Pimblott [54] chose G°( OH) = 5.50 molecules (100 eV) ... 

Ung (984) has used 02 as an H atom scavenger in the 1470 A photolysis of H20. The ratio of H2 with 02 present to the overall H2 yield without 02 is 23%. From this information Ung estimates that the primary yield of H2 production is 8%. 

Breckenridge and Taubc (144) have studied the photolysis of OCS + CS2 and OCS + N20 mixtures at 2288 and 2537 A. They have demonstrated that the primary yield of production ofS( D) [process (Vl-25a)] is 0.74 0.04 and 0.25 for S(3P) production [process (Vl-25b)] in agreement with the results of Gunning and Strausz (430). The deactivation process [process (VI-28)] must be about one third that of the total reaction of S( >) with OCS in order to be consistent with their finding that 50"- of the S atoms formed in the primary process react as S(3P)(144). 

The primary yield of dissociation at the low-pressure limit is unity at 3130 A, slightly less than unity, at 3340 A, and about 0.04 at 3660 A. The ( () values decrease at higher pressures of ketenc or added inert gas and the rate of decrease is much larger at longer wavelengths. The dissociation lifetimes arc 0.3, 4, and 850 nsec, respectively, at 3130, 3340, and 3660 A [Porter and Connelly (814)]. 

Palmer and Lossing (73) obtained analogous results with isobutane. Thus for iso-CiHio, the relative primary yields of terl-butyl to isobutyl radicals were found to be in the ratio, 7 1. However, when the tertiary hydrogen atom in the molecule was replaced by deuterium this ratio dropped to about 1 2. 

It will now be instructive to examine the n-butane reaction (76). In this case the reaction follows almost exclusively a single path leading to the formation of sec-butyl radicals. The percentage of the quenching done by the two methylene groups is very nearly the same as that for the tertiary C-H bond in isobutane (i.e. >90%). However, the primary yield of w-butyl radicals ( 2%) from w-butane is decidedly less than that for isobutyl radicals ( " 14%) from isobutane. This behavior can be readily interpreted on the basis of a cyclic transition-state structure, but not with an open-chain transition state. For the two reaction sequences, we may write ... 

If the carbon monoxide quantum yield is indeed unity and the primary yield is essentially unity, the following processes may be neglected unless the intensity is extremely high (105), (106), (109). Since (107) does not decrease the total number of radicals in the system, (104) and (108) are the radical removing steps and... 

It must be re-emphasized, however, that a true continuum in a spectrum is more difficult to identify than for diatomic molecules, and a primary yield of unity independent of total pressure and of scavengers is better proof of the presence of a continuum than is the appearance of the spectrum. 

The Consultative Committee for Amount of Substance (CCQM) has set up a definition of primary methods [1, 2] and has selected some methods with the potential of being primary , from the viewpoint of the end user. From the point of view of metrology, methods used for linking the chemical measurements with the SI system at the highest level should not refer to other amount of substance standards. This requirement excludes methods which are relative in their principle. Some other methods identified as having the potential to be primary yield information expressed as amount fraction. This is essential for evaluation of purity, but in order to convert it to a value useful for transfer of the unit, additional information on the identity (molar mass) and content of the impurities is required. This additional information is needed to convert the result into amount content or similar quantities. 

Cobut V, Corbel C, Patau JP. (2005) Influence of the pH on molecular hydrogen primary yields in He2-t ion tracks in liquid water. A Monte Carlo study. Rad Phys Chem 72(2-3) 207-215. 

In azo-dye-based laminar photoreceptor systems, neither the dissociation efficiency nor the primary yield of bound pairs is dependent on excitation wavelength [11 13]. There is a strong positive linear correlation between the dissociation efficiency and the peak value of the dipole moment associated with the transition between the ground and first excited states in the azo photosensitizer (obtained from electroabsorption data) [14]. It is likely that in these systems the efficiency of dissociation is constrained by the electron transport process away from the interface into the bulk of the sensitizer material, because transport of holes through the donor matrix is much more rapid. 

As only those alcohols which are primary yield aldehydes so also only those which are secondary yield ketones. The relation in composition between the secondary propyl alcohol and acetone is the same as between ethyl alcohol and acetaldehyde. What then is the constitution of ketones and why do secondary alcohols not oxidize to aldehydes but to ketones ... 

Benzyl alcohol or phenyl methyl alcohol given above can, of course, exist only as a primary alcohol there being only one carbon group in the side chain. Like the aliphatic alcohols, the primary yield aldehydes and then acids on oxidation, the secondary yield ketones and the tertiary break down. 

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