Evidence of saddle point ionization mechanism in positron-atom collisions

DELLA PICCA, RENATA; FIOL, JUAN; BARRACHINA, RAUL OSCAR

Rosario, Argentina

Conferencia; 24 International Conference on Photonic, Electronic and Atomic Collisions (ICPEAC); 2005

For many years, the validity of a novel ionization mechanism in ion-atom collisions was substantiated by claims of theoretical and experimental evidence, but also shadowed by doubt and skepticism. The basic idea is that an electron could emerge from the collision by travelling free-of-forces in the sadd le-point of the potential produced by the pro jectile of charge ZP and the residual target-ion of charge ZT . For an ionic projectile, this condition would lead to the appearance of some structure in the electron spectra in the forward direction for electron velocities  vs = vP /(1 + sqrt(ZP /ZT) ), where vP is the relative velocity between the pro jectile and the target.    In this communication we search for evidence of saddle-point electrons in the ionization of atoms and molecules by positron impact. To this end we calculate the quintuple differential cross section (QDCS) by means of a continuum distorted wave approximation that employs the correct kinematics. These calculations show structures that seem to be consistent with a Thomas – like double-collision mechanism leading to a saddle-point final state. We find a clear structure located at precisely the energyand emission angle where a saddle-point electron is supposed to appear. For instance, unmistakable signatures of these structures can be observed in the QDCS as a function of the electron (a), the positron (b), or the recoil-target-ion (R) momenta, when the electron and positron angles are constrained by the condition theta− = theta+ . We also analyze how these structures behave as a function of the mass, charge and velocity of the pro jectile, by considering other muonic and ionic species. We find that the evidence is convincing but not conclusive. In particular, we observe that while the saddle-point mechanism is known to be important at relatively low incident velocities for ion impact, the structure reported in this work is still present for initial projectile energies as high as 1000 eV.