Ultrafast reactions of energetic species produced by ionizing radiation.

As an ionizing radiation particle passes though water, it leaves in its wake clusters of low-energy electrons, and highly reactive molecular ions and excited states. The relative spatial distribution of these clusters depends upon the nature and the velocity of the ionizing radiation. For instance for energetic electrons the clusters are well separated (low-LET), while for alpha particles the clusters overlap (high-LET). The physical, physico-chemical and chemical processes that occur on a sub-picosecond time scale in the track of clusters are not well characterized. The principal questions concern (i) the attenuation, reaction, trapping, thermalization and solvation of low-energy electrons, and (ii) the fates of the excited states of water, the molecular cation, H_2O^+, and the cluster anions, (H_2O)_n^-, in the liquid phase. There is information on some of these processes in the gas phase, and limited data for water ice and water clusters, but essentially nothing is known for bulk liquid water: Photochemical studies of the dynamics of the precursors to the hydrated electron have identified several different states. In addition, radiation chemical studies have demonstrated that precursors to the hydrated electron play a dominant role in the sub-picosecond formation of H2 in both low-LET and high-LET radiolysis. A dissociative electron recombination reaction involving a second radiation-induced transient, either the radical cation H_2O^+ or the pre-hydrated H_3O^+ ion, has been identified as the dominant source of sub-picosecond H2 in high-LET radiolysis of water; however, other mechanisms cannot be discounted as contributing pathways in low-LET radiolysis. The chemistry of H_2O^+, H_2O* and (H_2O)_n^- in water is not only significant in the production of H_2, these species also represent the precursors to the oxidizing radicals, OH (O^-), O and HO_2, and the molecular species, O2 and H_2O_2. The yields of OH and other reactive oxygen species have been traditionally hard to quantify. To demonstrate the significance of the short-lived precursor species, their reactions in low- and high-LET radiolysis will be compared and contrasted and their influence on observed radiation effects will be discussed .