Merged-beams experiments in atomic and molecular physics

The merged-beams technique is powerful for the experimental study of certai n classes of atomic and molecular processes that cannot be as readily or ac curately addressed by other methods. The principal advantages of the techni que are the ability to make quantitative studies of collisional interaction s with high resolution at low relative energies, to collect products that h ave undergone appreciable angular scattering, and to investigate processes involving short-lived or chemically-reactive species. Despite continuing ad vances in ion-source and particle-beam technologies, merged-beams experimen ts remain a challenge, constituting a relatively small but growing fraction of the worldwide effort in atomic and molecular collisions research. This review outlines the fundamental principles of the merged-beams method, revi ews techniques and progress, and focuses on three active programs to highli ght the advantages of the method for addressing fundamental questions in at omic and molecular physics.