Collision-Induced Dissociation of Diatomic Ions

It is the aim of this chapter to show how, in recent years, relatively simple mass spectrometry experiments have contributed considerably to the general understanding of dissociation in heavy-particle collisions. When a well-defined primary beam of (diatomic) molecular ions impinges on a gas target at the object point of a mass spectrometer, this instrument can be used to measure the laboratory momentum distribution of the charged fragments which originate from dissociative collisions. This laboratory distribution can be converted into a momentum distribution in the center-of-mass coordinate system of the parent molecule, provided some requirements are met. In a dissociative collision of a diatomic molecule and an atom, taking into account conservation of energy and momentum, five and in many cases six velocity components of the separating particles should be measured simultaneously in order to describe the collision completely. Since only the momentum distribution of the charged fragment is determined, the interpretation of the measurements requires a number of basic concepts and models in the first place. Moreover, certain rather stringent requirements should be met in order to extract meaningful conclusions from the data. The most important of these requirements is that the laboratory deflection of the molecular center of mass may be neglected with respect to the laboratory deflection of the fragment ions which results from their dissociation velocity component in the molecular center-of-mass system.