Vortex structures in atomic ionization processes have been the subject of recent studies. In positron-hydrogen [Navarrete, F. O. et al, J. Phys. B, 48 (5), 055201, 2015] and proton–-helium [Guarda, T. A. et al, Atoms 13, 3, 2025], the appearance of a vortex in the vicinity of the electron capture peak in momentum space has been observed. Although the exact position has not been completely elucidated, the coincidence between the angles pointing the vortex and the angle corresponding to one of the Thomas mechanisms—describing kinematic conditions for electron capture—is remarkable. This apparent connection is not completely understood yet.

In order to shine some light upon this mystery, we have carried out several calculations of ionisation differential cross-sections of hydrogen-like targets, using projectiles of different masses. Using an automated algorithm to find the position of the vortices in momentum space, we can compare the angles arising from the different kinds of Thomas mechanisms and the saddle point position.