Electron Vortices in Femtosecond Multiphoton Ionization.

Multiphoton ionization of potassium atoms with a sequence of two counter-rotating circularly polarized femtosecond laser pulses produces vortex-shaped photoelectron momentum distributions in the polarization plane describing Archimedean spirals. The pulse sequences are produced by polarization shaping and the three-dimensional photoelectron distributions are tomographically reconstructed from velocity map imaging measurements. We show that perturbative ionization leads to electron vortices with c(6) rotational symmetry. A change from c(6) to c(4) rotational symmetry of the vortices is demonstrated for nonperturbative interaction.