Positive-ion scavengers

Enhancement of Ps formation. As expected from the spur model, all solutes that are efficient hole scavengers, thus somehow preventing the recombination process and increasing the electron availability (see reactions I—IX) enhance Ps formation. In water, strong positive ion scavengers are essentially the halide and pseudo-halide ions, together with amines. A convenient empirical equation to describe the Ps intensity variation is as follows [2] ... 

In Fig. 4 the after-pulse growth of the PR-TRMC transients due to positive ion scavenging by different Tt-bond conjugated polymers in oxygen saturated benzene is illustrated. From kinetic fits to the transients, using the known free-ion yield in benzene, the intrachain hole mobility could be determined and the values were found to vary from a low of 0.02 cm /Vs for the polythiophene to a high of 0.74 cm /Vs for the polyfluorene derivative. Mobility values in the... 

Warman JM, de Leng HC, de Haas MP, Anisimov OA. (1990) Positive ion scavenging by olefins in trans-decalin TRMC and product analysis studies. Radiat Phys Chem 36 185-190. 

Table II. Effect of Electron Scavengers on Positive Ion Scavenging by Cyclopropane in Cyclohexane...

The product yield from positive-ion scavenging studies has been known for some time to be approximately proportional to the square root of the solute concentration, and recently it has been found to be well described by an empirical expression identical in form to Equation II over the concentration range 10"4 to 10"2Af (product yields 0.13 to 0.30) (5 4)—i.e.,... 

It is much more difficult to find good positive-ion scavengers than electron scavengers. In ammonia and in hydrocarbons, cyclopropane may be used for this purpose. 

Physicochemical molecular ionization positive ion scavengers transfer of one electron to polymer cation without subsequent excitation... 

Emphasis will also be placed on approaches which lead to the removal of reactive species from the gas phase as well as the special role of energetic positive ions in plasma-surface interactions. Controlled scavenging of critical species from the gas phase and/or at specific surfaces and the degree of positive ion bombardment at a given surface can in fact result in simultaneous polymerizatidn at one surface and etching at another within the same apparatus. 

It is probable that all scavengers may capture electrons, which in the absence of scavenger would not escape the parent positive ions (G = 2-3), in addition to those electrons which have escaped the positive ion (G a 0.1). Whereas the products of electron capture by most solutes still undergo charge neutralisation with neighbouring positive ions, it is assumed that N20 decomposes before charge neutralisation can take place. 

Rzad and Schuler" studied the radiation chemistry of a solution of " C-cyclopropane in hexane over the concentration range 10 " to 10 M. The main radioactive products, which appear to result from ion molecule reactions, are propane formed by H2 transfer (50 %) and by H transfer (20 %) and mixed nonanes (30 %) formed by the addition of CaHg unit to a hexyl ion. At the lower concentrations, very pronounced dose dependence of the yields was observed. This was ascribed to a competitive formation of olefins in the radiolysis. For cyclopropane-cyclohexane solutions the chemical processes seem to be considerably more complicated. The observed yield of total radioactive products extrapolated to zero concentration of cyclopropane are 0.05 and 0.11 G units for hexane and cyclohexane, respectively. These limiting yields are of the order of magnitude of and appear to be related to, the free ion yields in these systems. Since cyclopropane was found to react with hydrocarbon ions" it is used quite often as a scavenger for positive ions, as in the work of Davids and coworkers . 

In the one experimental study on electron scavenging carried out with high LET radiations Burns and Reed (8) have examined the yields of N2 and H2 produced from nitrous oxide-cyclohexane solutions by 2 Mev. helium ions (LET — 20 e.v./A.). At particular N20 concentrations they find significantly lower N2 yields and smaller decreases in the H2 than observed in the y-ray experiments (at 0.1 M N20 G(N2) = 0.6 in experiments with helium ions vs. 3.8 for those with y-rays AG(H2) = 0.2 vs. 2.3). Here reaction between the electron and a positive ion other than its original partner apparently becomes significant, and the electronscavenging process cannot compete as efficiently as at low LET s. 

Whether the electrons are free or not, their ultimate fate in the absence of an electron scavenger will be recombination with a positive ion. 

One further comment can be made. The hydrogen yield observed for cyclohexane solutions 0.1 M in CH3I is 2.5 (65). This is lower (by 0.8) than the yield observed at similar concentrations of other electron scavengers (Figure 3). The total G(H2) + G(CH 3) 1 Gr(CH4)UnRcaveiiKeabl equals 6.0 which can be compared with the slightly lower totals indicated in Table IV for methyl chloride solutions. Thus, while studies of the effect of methyl iodide on the positive-ion reactions of cyclopropane indicate that the methyl iodide does undergo positive-ion reactions, these reactions do not seem to make more than a relatively minor contribution (possibly — 0.4) to the products under discussion. 

In the y-radiolysis of water-isopropyl alcohol mixtures in the vapor phase, the total yields of hydrogen, methane, and carbon monoxide are linear over the whole concentration range, in the presence and absence of electron scavengers. The majority of the hydrogen produced from energy absorption by both water and isopropyl alcohol is formed by H-atom abstraction from isopropyl alcohol. Part of the H-atom yield is formed by electron-positive ion neutralization and part by processes not involving electrons. As far as the formation of H atoms is concerned, both electron-positive ion and ion-ion neutralizations appear to be independent of the composition of the positive ion cluster. A yield of molecular hydrogen is also present in both water and isopropyl alcohol. 

Despite the changing composition of this positive ion cluster with changes in /w and /p, G(H2)t is linear with isopropyl alcohol concentration both in the presence and absence of electron scavengers (Figure 1). Thus, there is no evidence that the composition of the positive ion affects either the conversion of electrons to H atoms via Reaction 9, or ion-ion neutralization in the presence of electron scavengers. This, however, is not completely unambiguous, particularly in view of the numerical similarities between G(H2)W° and G(H2)P° in the presence and absence of electron scavengers. 

Since much of the methane yield appears to be produced via methyl radicals, the yield should be affected by propylene concentration. We found little effect below 1 mole % propylene but the yield fell by 20% as the propylene concentration increased to 10 mole %, both in the presence and absence of SFe. These results, however, do not indicate how much methane is formed from CH3 precursors and how much by molecular processes. The effect of propylene is being studied further in an attempt to obtain this information. The electron scavengers appear to increase the methane yield slightly. Since they do not decrease the yield, this probably means that electron-positive ion neutralization does not produce methane. The reason for the increase in G(CH4) is not clear but it could occur if ion-ion neutralization produced methane, or if some physical interaction caused a transfer of energy from the additives to isopropyl alcohol. The slight increase in CH4 yield with SF concentration tends to favor the latter assumption. 

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