Laser-assisted electron and positronium collisions*

Various electron, positron and positronium collision processes can be strongly enhanced by infrared fields of moderate intensities. These include laser-assisted bremsstrahlung, laser-assisted radiative recombination of electrons/positrons with protons/antiprotons, and laser-assisted positronium formation in positron-atom collisions. The enhancement mechanism in the case of the Coulomb interaction between the projectile and the target is called the Coulomb focusing. It occurs in a very broad range of impact parameters due to the electron quiver motion: the electron performs several oscillations which allows a longer action of the Coulomb field pulling the electron closer to the Coulomb center. A similar mechanism plays role in positron collisions with the hydrogen atom in an excited state when the atom possesses a nonzero dipole moment. In this case we obtain the dipolar focusing effect leading to an enhancement of the positronium formation. The field-free positronium collisions with atoms and molecules are of a great interest due to their similarity with electron collisions discovered experimentally by the University College London group [1]. Our calculations confirm this findings for a number of atomic and molecular targets including resonance collisions involving N₂ and CO₂ molecules.

[1] S. J. Brawley, S. Armitage, J. Beale, D. E. Leslie, A. I. Williams, and G. Laricchia, Science 330, 789 (2010).