Chapter 5 - Radiation Chemistry of Gases

Abstract

This chapter provides an overview of the gas-phase radiation chemistry. For condensed phases, two important aspects of gas-phase radiolysis are the analysis of molecular products and the effect of added scavengers on these products. Ion–molecule reactions involve a positive ion and a neutral molecule, frequently the parent molecule. Historically, there has been a dichotomy in the interpretation of the radiation-chemical yields in hydrocarbon gases. The process of fragmentation refers both to the parent and product ions. Each fragmentation mode has a critical energy requirement called the appearance potential. The fragmentation pattern depends critically on the excess energy over the highest relevant appearance potential. The dissociation processes ensuing from excited alkane molecules may be classified as elimination of H, H2, CH2, or entire alkane molecules. There is some analogy with the ionic fragmentation process. Also, there is an analogy in certain cases where the dissociation product has enough energy to undergo further dissociation. Neutralization is considered as a poorly understood process in the gas phase. While in the best possible cases, it may be considered as a homogeneous second-order process. Gas-phase radiolysis can sometimes result in chain reactions involving H atoms or other radicals. The chapter provides two useful theoretical aspects in the radiation chemistry of gases—the quasi-equilibrium theory (QET) and ion-molecule reaction.