Dipolar focusing in laser-assisted positronium formation

We use the Classical Trajectory Monte Carlo method to calculate positronium (Ps) formation in the $e^+ + {\rm H}(n)$ collisions for $n\geq 3$ in the presence of an infrared laser field. This process is assisted by the dipolar focusing due to nonzero dipole moment of the hydrogen atom in excited states. The effect is similar to the Coulomb focusing effect in radiative recombination [1] and bremsstrahlung [2] in the presence of an infrared laser field. The dipolar focusing effect makes positron approach the target even for large impact parameter. This effect significantly enhances the Ps formation cross section which appears to be infinite for certain values of the laser field phase. A similar effect can occur in collisions of positrons with other atoms in excited states which can lead to improvements in efficiency of Ps formation.

[1] I. I. Fabrikant and H. B Ambalampitiya, Phys. Rev. A 101, 053401 (2020).

[2] H. B Ambalampitiya and I. I. Fabrikant, Phys. Rev. A 99, 063404 (2019).