Angle-dependence of ion kinetic energy spectra obtained by using mass spectrometers I. Theoretical consequences of conservation laws for collisions

Abstract

Collision-induced processes for ion beams in the kilovolt energy range have been studied in mass spectrometers since the inception of the technique (‘Aston bands’). In most such studies to date, the fast collision products bave been collected over a range of angles relative to the initial projectile trajectory, this range being defined by the ion optics of the instrument used. More recently, there has been some interest in controlling and exploiting the collection angle for the fast collision products. The motivation behind such studies may be expressed qualitatively in terms of the more violent collisions favouring higher-energy processes, and also resulting in a larger scattering angle for the projectile ion in the activating collision. Thus, it might be hoped that collection angle could serve as an experimental parameter whose variation controls the energetics of the collision-induced processes actually observed. The present work examines the probable limitations of such an approach, on the basis of classical collision theory. Even without knowledge of an appropriate potential function for the projectile ion-neutral target interaction, it is possible to obtain useful quantitative information concerning such collisions. The extensive work on monatomic projectiles and targets is reviewed, and an attempt made to extend these results to polyatomic species of more interest to the mass spectrometrist. When the collision induced process to be studied is chemical fragmentation, the observed angular distribution is a convolution of two effects of comparable importance, namely the scattering angle-energy deposition relation which is the desired result of the experiment, and the internal energy of the projectile (precursor) ion released as excess translational energy of the fragments. The most recent experimental work on angle-resolved mass spectrometry is critically discussed in the light of these considerations.