The merged-beams technique is powerful for the experimental study of certain classes of atomic and molecular processes that cannot be as readily or accurately addressed by other methods. The principal advantages of the technique are the ability to make quantitative studies of collisional interactions with high resolution at low relative energies, to collect products that have undergone appreciable angular scattering, and to investigate processes involving short-lived or chemically-reactive species. Despite continuing advances in ion-source and particle-beam technologies, merged-beams experiments 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, reviews techniques and progress, and focuses on three active programs to highlight the advantages of the method for addressing fundamental questions in atomic and molecular physics.

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A new interference mechanism, analogous to "classic" double-slit electron scattering, has been identified in low-energy ion-atom collisions. This Coulomb "path" interference results from the existence of two trajectories, indistinguishable with respect to laboratory energy and emission angle, along which ejected autoionizing electrons may be scattered by the attractive Coulomb potential of the slowly receding spectator ion. A simple semiclassical description is presented. Calculated model line shapes are in excellent agreement with the strong angular dependence of the interference structure observed in the He-target 2s 2 l S autoionizing line shape measured near 0° following 10-keV He + + He collisions. PACS numbers: 34.50.Fa, 34.10.-hxIn low-energy ion-atom collisions, post-collision interaction (PCI) of autoionization electrons with the attractive Coulomb field of the slowly receding spectator ion leads to spectral line shapes which are broadened and shifted towards lower electron energies. In addition, those electrons emitted when the collision partners are still comparatively close together may suffer significant deflections as they emerge from this field. Such deflections result in further perturbation of the final energy distributions of the ejected electrons and, as recently noted, ] effectively compress the solid angle into which these electrons are emitted, resulting in an enhancement or "focusing" of intensity in the direction of the receding ion.The influence of the Coulomb field on the electron's classical trajectory is illustrated in Fig. 1 for low-energy He" 1 " + He collisions. Autoionizing states (He**) formed in the collision may decay at a time / when the collision partners are separated by a distance R(t) =vt + S. The key observation is that, for a localized source of autoionizing electrons and an attractive potential, there are different paths (corresponding to slightly different emission times) by which the electrons may emerge from the FIG. 1. Effect of the spectator ion's Coulomb field on the classical trajectories of ejected target autoionizing electrons following low-energy He + 4-He collisions. collision with a given laboratory energy E and emission angle 0. Two such trajectories (denoted A and B) are illustrated in Fig. 1, where v and 0 are the asymptotic electron velocity and emission angle after scattering. Since A and B represent two indistinguishable paths to the same final electronic state, interference may occur. This Coulomb "path" interference thus represents an ion-atom collision analog to "classic" double-slit electron scattering. A similar interference phenomenon has been previously observed in the form of "rainbow ghosts" in near-side-far-side nuclear scattering by Satchler et al. 2 In this Letter, we present evidence for Coulomb path interference in low-energy ion-atom collisions. This mechanism manifests itself in the form of an anomalous feature in the Coulomb-focused PCI-broadened line shape of the target 2s 2 l S autoionizing state produced in 10-keV He + + He collisi...

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