Radiation Chemistry 1968-1976
Radiation Chemistry concerns the interaction of ionizing radiation with matter. Ionizing radiation used for these investigations includes ultraviolet radiation, vacuum ultraviolet radiation, x-rays, gamma rays, and energetic electrons from accelerators. While radiation chemistry is a unique field, there are significant similarities between radiation chemistry and photochemistry where absorption of lower-energy (non-ionizing) photons produces (mostly) neutral species. Aside from the intrinsic importance of basic research in radiation chemistry, applications include the understanding of the unique chemistry of high-radiation environments. In addition, radiation chemistry provides access to species in unusual oxidation states; molecules, radicals and ions that are otherwise difficult to produce in quantities sufficient for study, some of which are of biological importance.
Ionization sources during this period included a 2-MeV Van de Graaff accelerator (first installed in the late 1940s and still in use today!), a 2-MeV Febetron accelerator for electron pulses of a ~25 ns duration, and 60Co sources.
The amount of research conducted in this field in the Chemistry Department during this period is no less than prodigious; we can only cover some of the highlights, and apologies are offered to those whose work is not cited here.
Properties of H2O3 and HO2
Since many chemical and biochemical reactions take place in aqueous solution, the radiation chemistry of water is of interest. Radiation chemistry research at Brookhaven played an important role in identifying the oxidizing and reducing radicals formed in water irradiation. Earlier,1 the Radiation Chemistry Group in the Chemistry Department demonstrated that the predominant reducing species has a negative charge and is a hydrated electron. One of the most notable accomplishments of this program in the sixties and the seventies was an ongoing investigation to further identify and characterize the reactive species in the radiation chemistry of water.
When air-saturated water undergoes pulse radiolysis, unusual radicals are formed. Water and hydrogen peroxide are common stable compounds of hydrogen and oxygen. However, the "next" species in the series is hydrogen sesquioxide, H2O3, believed to be formed by the reaction, OH + HO2 ® H2O3. The absorption spectrum of H2O3 was observed below 280 nm, its half-life was measured, and the activation energy for its decomposition at pH 2 was determined to be 16.5 kcal/mol. Previous studies indicated that the perhydroxyl radical HO2 exists in three forms: O2-, HO2, and H2O2+. Studies revealed no evidence for the third form, H2O2+.2
1. "The Nature of the Reducing Radical in Water Radiolysis" G. Czapski and H.A. Schwarz, J. Phys. Chem. 66 471 (1962).
2. "The Absorption Spectra and Kinetics of Hydrogen Sesquioxide and the Perhydroxyl Radical" B.H.J. Bielski and H.A. Schwarz, J. Phys. Chem. 72 3836 (1968).
The Spur Model of Water Radiolysis
The spur model of water radiolysis holds that many observed effects, including the formation of the molecular products hydrogen and hydrogen peroxide, are due to the inhomogeneous distribution of the radiation-produced free radicals, which are formed in groups called spurs. Subsequent reactions of these radicals, while diffusing in the bulk of the solution produce the observed effects. Earlier tests of the model in the Chemistry Department demonstrated that it was consistent with almost all known facts of water radiolysis, but that about one-third of the molecular hydrogen was formed in some other process. Others pointed out that, in all such calculations, a unique radius is assumed for spurs containing any given number of radicals. It was shown3 that the shape of the curve relating molecular product formation and the concentration of solutes capable of reacting with the radicals is essentially unchanged by assuming a statistical distribution of spur sizes.
3. "Application of the Spur Diffusion Model to the Radiation Chemistry of Aqueous Solutions" H.A. Schwarz, J. Phys. Chem. 73 1928 (1969).
Mobility of Electron in Hydrocarbons
Until this time, the only charge carriers observed in irradiated hydrocarbons were ions of mobility about 0.001 cm2/(V-s). Using a novel method, it was observed that in high-purity tetramethylsilane and eight other hydrocarbons that electron exhibited mobilities ranging from 0.09 to 90 cm2/(V-s). This extraordinary result led to an ongoing study of electron mobility in nonpolar solvents that continues today.4
4. "Ranges of Photoinjected Electron in Dielectric Liquids" R.A. Holroyd, B.K. Dietrich and H.A. Schwarz, J.Phys. Chem. 76 3794 (1972).
Last Modified: June 28, 2012