Source of Molecular Hydrogen in High-Temperature Water Radiolysis

Molecular hydrogen is a primary product of the interaction of low-LET (gamma, beta) radiation with water, and previous measurements have shown that its initial yield increases at elevated temperature. This has been the subject of controversy because more atomic H and (e(-))(aq) free radicals escape recombination at elevated temperature, and the corresponding production of H-2 should decrease. Room temperature experiments have demonstrated that a large fraction of H-2 also comes from early physicochemical processes (presumably electron-hole charge recombination and/or dissociative electron attachment), which can be suppressed by scavenging presolvated electrons. In the present work we extend these scavenging measurements up to 350 degrees C to investigate why the H-2 yield increases. We find that most of the H-2 yield increase is due to the presolvation processes. Relatively small changes in the scavenging efficiency vs LET, and a significant effect of temperature depending on the (positive or negative) charge of the scavenger, indicate that the presolvation H-2 is dominated by electron hole charge recombination rather than dissociative electron attachment at all temperatures.