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Solvated electron yield in the radiolysis of formamide Head, David Alan


Some of the thermal electrons formed when a liquid is subjected to high-energy radiations eventually become solvated. These solvated electrons (e⁻solv) are of special chemical interest due to their powerful reducing nature. The mechanism by which solvation occurs is not well understood, but must depend upon several solvent properties, of which the static dielectric constant is probably the most important. Some insight into the factors determining electron solvation may be gained through a knowledge of the radiation yields of e⁻solv in a wide variety of liquids. Since water is the liquid of highest dielectric constant (є=80) studied to date it seemed worthwhile to extend our knowledge beyond this point. Of the four common liquids with dielectric constants greater than that of water, formamide (є=109) was best suited for this study. In the present work the e⁻solv yield in formamide was found to be only slightly greater than its yield in water, which is surprising considering the large increase in dielectric constant. A modified correlation which, in addition to the dielectric constant, included the solvating ability and the stopping power of the solvent was shown to give a better fit to data for a number of liquids (resolving, for instance, the ammonia anomaly), indicating that these factors also govern the yield of e⁻solv. The yields of hydrogen, carbon monoxide, and nitrogen formed when formamide and its nitrous oxide solutions were irradiated with ⁶⁰Co Ƴ-rays gave information concerning the e⁻solv yield. Using the electron scavenger N₂O and the scavenger pairs N₂O/Cd⁺⁺, N₂O/Ag⁺, and N₂O/acid, the number of e⁻solv formed per 100 eV of energy absorbed was found to be 2.9±0.1. This yield corresponds fairly closely with the value which can be calculated using Freeman's non-homogeneous model. The primary yields of H₂ HCONH*₂, CO, and HCO were evaluated as .75±0.05, 0.4±0.1, ≤ 0.55±0.07, and ≥ 1.15±0.20 molecules/l00eV respectively. At low doses (up to ∼5xl0¹⁹ eV/ml) the yields of H₂, N₂, and CO were dose independent for the dose rate of ∼4xl0¹⁷ eV/ml-minute used. In two types of pulse radiolysis experiments light absorption was not observed in the 4500 Å to 8500 Å wavelength region contrary to expectations. A comparison of these results with those from other work may be interpreted as suggesting that the half-life of formation of e⁻solv, in formamide is between 0.36 and 1.9 microseconds. This tentative conclusion, if correct, would mean that e⁻solv was a secondary, not a primary, product in formamide and this would be of general significance to the primary processes of radiation chemistry.

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