title: Electron Vortices in Femtosecond Multiphoton Ionization.
abstract: 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.
authors.affiliation: Carl von Ossietzky Universität Oldenburg, Institut für Physik, Carl-von-Ossietzky-Straße 9-11, D-26129 Oldenburg, Germany.
authors.forename: D
authors.initials: D
authors.lastname: Pengel
authors.affiliation: Carl von Ossietzky Universität Oldenburg, Institut für Physik, Carl-von-Ossietzky-Straße 9-11, D-26129 Oldenburg, Germany.
authors.forename: S
authors.initials: S
authors.lastname: Kerbstadt
authors.affiliation: Carl von Ossietzky Universität Oldenburg, Institut für Physik, Carl-von-Ossietzky-Straße 9-11, D-26129 Oldenburg, Germany.
authors.forename: D
authors.initials: D
authors.lastname: Johannmeyer
authors.affiliation: Carl von Ossietzky Universität Oldenburg, Institut für Physik, Carl-von-Ossietzky-Straße 9-11, D-26129 Oldenburg, Germany.
authors.forename: L
authors.initials: L
authors.lastname: Englert
authors.affiliation: Carl von Ossietzky Universität Oldenburg, Institut für Physik, Carl-von-Ossietzky-Straße 9-11, D-26129 Oldenburg, Germany.
authors.forename: T
authors.initials: T
authors.lastname: Bayer
authors.affiliation: Carl von Ossietzky Universität Oldenburg, Institut für Physik, Carl-von-Ossietzky-Straße 9-11, D-26129 Oldenburg, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Wollenhaupt