RECENT PROGRESS IN CHEMI-IONIZATION KINETICS

Abstract

A review of chemi-ionization reactions based on knowledge acquired since 1968 is given. Chemi-ionization reactions are considered to be: ‘Reactions by which the number of elementary charge carriers is increased as the direct result of the formation of new chemical bonds’. This includes both associative (A + B → AB+ + e-) and rearrangement (A + BC → AB+ + C + e-) ionization reactions. Rate coefficients for these reactions are often on the order of 1012 to 10-9 ml molecule-1 sec-1 and decrease with increasing temperature (relative collision velocity). For a given reactant A the rate coefficients also tend to decrease with an increasing number of atoms in B. A number of examples of exoergic chemi-ionization reactions between ground state reactants have now been definitely established. More detailed knowledge of reactions involving an electronically excited collision partner has become available and meaningful comparisons to Penning ionization can be made. However, our understanding of the details of the processes occurring are based primarily on observations of noble gas metastables, which, as is shown, do not in all respects apply to reactions of other species.