Multiparticle Imaging of Fast Molecular Ion Beams

Abstract

For many years, two-dimensional (2D) and three-dimensional (3D) fragment imaging techniques have been successfully used in the study of molecular structure [1] and for the study of the dynamics of various molecular dissociation processes, such as photodissociation [2] , dissociative recombination [3], atom—molecule collision-induced dissociation [4] , and dissociative charge exchange [5] . For fast molecular ion beams (in the present context, fast means kinetic energies in the range of keV to several MeV), the basic experimental scheme includes the induced dissociation of a single molecule from the beam, and the fully correlated measurement of the asymptotic velocity vectors of the outgoing atomic and molecular fragments. If the initial velocity of the molecule is large, then all the fragments will be projected into a cone defined by the ratio of their transverse velocities and the initial beam velocity. In such a case, the transverse velocities are deduced from the 2D position on the surface of a position—sensitive detector, while the longitudinal velocities can be derived from the (relative) time of arrival at the detector. The specific physical information provided by the images depends on the particular dissociation process. In general, one obtains information about the initial molecular quantum state prior to the dissociation, the final state of the fragments and about the dynamics of the reaction, such as angular dependence, kinetic energy release or potential curves.