H atoms, yield

Misik V, Riesz P (1997) Effect of Cd2+ on the H atom yield in the sonolysis of water. Evidence against the formation of hydrated electrons. J Phys Chem A 101(8) 1441-1444... 

The model predicts a very small initial H atom yield (0.62), smaller than the corresponding yield in the vapor phase. It may be conjectured that some of the incipient H atoms convert to eh in the liquid phase, but the mechanism is unknown. 

Hydrated electron yields decrease with increasing MZ jE, but they do not seem to decrease to zero. Experiments have been performed on aerated and deaerated Fricke dosimeter solutions using Ni and ions [93]. One half of the difference in the ferric ion yields of these two systems is equal to the H atom yield. The Fricke dosimeter is highly acidic so the electrons are converted to H atoms and to a first approximation the initial H atom yield can be assumed to be zero (see below). There is considerable scatter in the data of the very heavy ions, but they seem to indicate that hydrated electron yields decrease to a lower limit of about 0.1 electron/100 eV. The hydrated electron distribution is wider than that of the other water products because of the delocalization due to solvation. This dispersion probably allows some hydrated electrons to escape the heavy ion track at even the highest value of MZ jE. 

Only two studies have determined H atom yields in neutral water with heavy ions [26,121]. H atom yields are extremely difficult to determine directly because of competition with hydrated electrons or OH radicals. Yields of H atoms are usually estimated from differences in molecular hydrogen yields using various scavengers for the H atom. Fig. 11 gives the H atom yields as a function of MZ jE. There are not many data points, but they seem to agree with each other and show a decrease with increasing MZ jE values. 

The measured H atom G-value is about 0.25 at MZ jE = 1, while the equivalent yield of hydrated electrons is found at MZ jE = 10. The persistence of the hydrated electron to higher MZ jE values suggests that it does not decrease to zero at an infinite value of MZ jE. Most H atoms are produced in conjunction with OH radicals in the core of the heavy ion track. The recombination rate constant is high so there is a small probability that H atoms will escape the track at high LET (MZ jE). H atoms can be formed by hydrated electron reactions and their yield cannot decrease to zero if hydrated electron yields do not. However, hydrated electron yields are low at high MZ /E values so the H atom yield can be considered negligible in this region. 

In order to test the validity of the trap theory Kevan, Moorthy, and Weiss (32) compared the H atom yields in a number of different systems. 

Figure 2. Dependence of H atom yields on solute concentration in 7-irradiated frozen aqueous solutions at 77 °K.

Reaction 3 occurring in ice, when an add is present. It is to be inferred that the H atoms formed according to Reaction 1 and observed in ice at 4° K. with a yield of G(H) 0.4 contribute little, if any, to the H atom yields in the acids at 77 ° K. With certain modifications to be discussed below, the concept of the radiation-produced electron as one of the reactive intermediates in the radiolysis of frozen aqueous solutions explains many experimental results. 

Table II. Effect of Different Electron Scavengers on H Atom Yields in 7-lrradiated Frozen Aqueous Solutions at 77° K.

H atom yields are decreased markedly even by small concentrations ( O.OIM) of the acceptor... 

H atom yields are not affected even by a high concentration — (1 M) of the acceptor solute, dec Decomposition occurs on mixing the solutions, ppt Precipitation occurs on mixing the solutions. 

The behavior of perchlorate would at first appear to be anomalous. Thus although in this case, H-atoms are expected to be formed and stabilized in the same way as in S04-2, HP04 2, P04 3, and C03 2, yet it is found (Table II) that the H atom yield is decreased in the presence of electron scavengers. To explain this behavior Kevan, Moorthy, and Weiss, (32) suggested that here the H atoms must be formed in the hydration shell of the anion by reaction of the electron with H +, the concentration of which in the hydration shell could possibly be higher than in the bulk. An alternative mechanism has been recently proposed by Kevan (31), according to whom the electron reacts with the anion to give an... 

Rate Constants. The effect of electron scavengers on the H atom yields can be considered as a competition between Reactions 4 and 8 (S, electron scavenger). 

E, the efficiency of electron formation by the action of radiation, is the number of electrons formed per 100 e.v. of energy absorbed, and I is the dose rate in units of 100 e.v. 1-1 sec.-1 kT [(H20) ] is a term to account for the disappearance of the electrons in the absence of any scavengers the nature of this process is not relevant to the present discussion and will be considered later. The H atom yield is then given by ... 

Comparing the last two equations shows that, for any given concentration of HX -, the H atom yield will be progressively reduced with increasing concentration of the scavenger in agreement with experiment. From the last two equations it can also be deduced that... 

With Na2S04, Na2HP04, NasP04, and Na2C08 as solutes where the mechanism of hydrogen atom formation and stabilization is different, the observed linear dependence of the H atom yields of the solute concentration is as expected on the basis of the proposed mechanism. Thus, since the probability of forming an H,OH radical pair in the hydration shell of the anion (—e.g., by dissociation of an excited water molecule) would be proportional to the anion concentration, and sigce the stabilization of the H atom is postulated as the result of the reaction of the OH radical with the anion in whose hydration shell it is formed, it follows that the yield should be proportional to the solute concentration. 

Cd +1 Radical Ions. From the results given in Table IV it is found that the presence of Cd +2 in H 0A or NaHS04 solutions markedly depressed the H atom yield obtainable on 7-irradiation at 77 0 K., a behavior analogous to that of the other electron scavengers. It is thus apparent that the reaction... 

Pentavalent Chromium. The radiation-induced formation of unstable lower oxidation states is not limited to the cations. As an example of the anions, Cr04-2 or Cr207 2 have been investigated. In acid media, the presence of dichromate markedly decreases the H atom yields, while in alkali hydroxide solutions an absorption band (Xmax = 585 m/x) and a characteristic ESR signal both attributable to the trapped... 

G 2 (Figure 14) as found in irradiated alkaline ice. However, even at 4°K. the H atom yield in pure ice is only 0.4, while at 77° K., no H atoms are found. If, on the other hand, all the H atoms produced by Reaction 3 should combine to form H2 gas, the yield of the latter should be of the order of G(H2) = 1, whereas the observed yield of molecular hydrogen is only G = 0.1 (18). Further, a mere increase in the pH of the system to inhibit Reaction 30 does not result in the stabilization of the solvated electrons for it has been found (45) that aqueous Na2C08 or NH4OH solutions (which have pH >7) on 7-irradiation at 77° K. do not... 

H-atom yield from O-1-C2H4 measured using Lyman-a 711 resonance fluorescence... 

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