Radiolysis experiments, design

Under these conditions, the conversion e " to O" is essentially complete in 15 ns but the half-life of its protonation in reaction (14) is 8 ns so complete conversion to OH takes 40 ns. In pulse radiolysis experiments designed to measure reactions of OH one should take care, therefore, to choose conditions that allow this radical to be fully formed before the reaction of interest begins. The hydrogen atom reacts only slowly with N2O (k = 2.1 x 10 dm mor s" [5]) so that approximately 90% ofthe radicals available in N,O-saturated neutral solution are OH and the remaining 10% are H . 

The reaction-rate constant kjfP is a chemical constant characteristic of a compound P with general validity. It can be measured in laboratory experiments designed to isolate the effect of a single environmental factor j. Often, for practical reasons, it is determined only relative to that of a well-studied model compound with an absolute rate constant known for the same reaction. In case of slow reactions it is generally easy to measure absolute rate constants directly. For the study of fast reactions, sophisticated short-time measurements, such as pulse radiolysis or flash photolysis, typically combined with kinetic absorption spectroscopy or kinetic phosphorescent measurements, must be applied. 

In photochemistry, excited-state decay or back-reaction of the electron-hole pair may limit the efficiency of generating the electron-transfer intermediate of interest. Radiolysis experiments can often be designed to generate oxidizing or reducing equivalents exclusively. 

In designing pulse radiolysis experiments, the fundamental physics and chemistry of radiolysis must be kept in mind. The properties of the accelerators and other radiation sources described in this chapter vary over a wide range. A certain kind of experiment may call for a particular type of accelerator, whereas others may be accomplished by using almost any equipment. Equally important, of course, is the variety of experimental detection systems that are available at accelerator facilities. Some of the available methods include dc or microwave... 

P. Humrickhouse, P. Wilson, et al., Gamma Shield Design for the Supercritical Water Neutron Radiolysis Experiment, Transactions ofANS, 92, 161-162 (2005)... 

Radiation chemistry can he used to study reactions of free radicals and of metal ions in unusual valency states, including electron-transfer reactions. In some instances, radiation chemistry facilitates experiments that can not he studied hy photochemistry, owing to differences in the fundamental physical processes in the two methods. Procedures have heen developed to accurately determine radiolysis radical yields, and a variety of physical techniques have heen used to monitor reactions. In particular, aqueous radiation chemistry has heen extensively developed, and many free radicals can he generated in a controlled manner in aqueous solution. There are extensive literature resources for rate constants and for experimental design for a variety of radicals. 

Radiation Chemistry of Solvents Water. The successful design of a radiation chemistry experiment depends upon complete knowledge of the radiation chemistry of the solvent. It is the solvent that will determine the radicals initially present in an irradiated sample, and the fate of all these species needs to assessed. Among the first systems whose radiation chemistry was studied was water, both as liquid and vapor phase, as discussed by Gus Allen in The Story of the Radiation Chemistry of Water , contained in Early Developments in Radiation Chemistry (8), Water is the most thoroughly characterized solvent vis-a-vis radiation chemistry. So to illustrate the power of radiation chemical methods in the study of free radical reactions and electron-transfer reactions, I will focus on aqueous systems and hence the radiation chemistry of liquid water. Other solvents can be used when the radiation chemistry of the solvent is carefully considered as noted previously, Miller et al. (I) used pulse radiolysis of solutions in organic solvents for their landmark study showing the Marcus inversion in rate constants. 

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