Hyperfme Interactions 73(1992)27-32 27 TRANSFER AND IONIZATION IN COLLISIONS OF POSITRONS WITH ATOMS Jim McGUIRE* and Jack C. STRATON Kansas State University, Manhattan, KS 66506, USA Kinematic singularities in certain few-bodyreactions may provide special features that are both conceptuallysimple and observable. Particle transfer is such a reaction. At high velocities, it is characterizedby singularitiesthat are manifested by peaks and ridges in differential cross sections. For electrons captured by protons, some of these singularfeatureshave been observed.For capture by positrons,i.e. positronium formation, new features are predicted. In this presentation, we wish to consider a particular reaction where scattering by positrons will lead to physical understanding that cannot be obtained with scattering of either electrons or protons. The reaction is transfer ionization, i.e. a two-electron process in which one electron is transferred to the projectile and the other is ionized. This is a few-body process in which kinematic constraints normally lead to observable ridge structures in the velocity distributions of the ionized electrons. For impact by positrons, however, these structures may be significantly modified by interference of competing quantum mechanical amplitudes. Because of the condition in panicle transfer that two particles leave the scattering region together, electron capture in a one-step process is classically forbidden by conservation of energy and momentum at high collision velocities. Consequently, the simplest classical mechanism for panicle transfer is a two-step process as proposed by Thomas [1] in 1927. This is easily understood using fig. 1. Here, the entire collision is coplanar since particles 1 and 2 go off together. If all the masses and the incident velocity 12 are known, then there are six unknowns 12; 12f and 123 as defined by fig. 1. Conservation of momentum gives two equations of constraint for each collision. Conservation of overall energy gives a fifth constraint and conservation of energy in the intermediate state gives a sixth constraint. With six equations of constraint, all six unknowns may be completely determined. The allowed values of 12', 12f and 123 depend on the masses M 1, M 2 and M 3. For example, for p§ + H-o H + p+, it is easily verified that (in the notation of *Present address: Tulane University.New Orleans, LA 70118, USA ~) J.C. Baltzer AG, ScientificPublishingCompany