Dipolar focusing in laser-assisted positronium formation

We consider theoretically positronium formation in collisions of positrons with excited hydrogen atoms H($n$) in an infrared laser field. This process is assisted by the dipolar focusing effect: a positron moving in a superposition of a laser field and the dipolar field can approach the atomic target even if its trajectory starts with a very large impact parameter, leading to a significant enhancement of the Ps formation cross section. Classical trajectory Monte Carlo method, which is justified for $n\geq 3$, allows efficient calculation of this enhancement. A similar effect can occur in collisions of positrons with other atoms in excited states which can lead to improvements in efficiency of positronium formation.