Review lecture: Unimolecular and collision-induced reactions of high velocity ions

Abstract

It is possible to produce beams of high velocity ions, either positively or negatively charged and with a translational energy homogeneous to within 1 part in 104. The currents carried by such beams are typically of the order 10-9A. By using a magnetic field, it is possible to select ions of a particular mass: charge ratio from such beams and to study their fragmentation behaviour in field-free space. Fragmentation may occur unimolecularly or may be induced to occur by causing the ion beam to interact with a collision gas, or in other ways. The fragmentation products of such reactions that also travel with high velocity can be analysed by using a magnetic field (momentum analysis) or an electric field (translational energy analysis). The method provides information concerning the structures of ions and the kinetics, energetics and mechanisms of the reactions; the pattern of translational energy release during the fragmentation provides information on the partitioning of the available energy between internal and translational modes. By selecting reactions that occur over a limited range of rate constants, ions containing a narrow range of internal energies may be selected for study. In collision-induced reactions, part of the translational energy of the ion is converted into electronic excitation. Ions in excited electronic states can also be produced by photo-irradiation of the beam. Ions excited in these ways can be used to give further information concerning energy partitioning during fragmentation. Reactions involving a change in the sign of the ionic charge, or in the number of charges carried, can be induced to occur by interaction with a collision gas. They provide information on ionization energies and electron affinities and on long-lived electronically excited states.