Limitations in photoionization of helium by an extreme ultraviolet optical vortex

Photoelectron angular distributions from helium atoms are measured using the circularly polarized extreme ultraviolet (XUV) vortex produced by a helical undulator as the higher harmonics of its radiation. The XUV vortex has a helical wave front and carries orbital angular momentum as well as the spin angular momentum associated with its circular polarization. While the violation of the electric dipole transition rules has been predicted for interactions between vortices and atoms, the photoelectron angular distributions are well reproduced by assuming electric dipole transitions only. This observation can be explained by the localized nature of the helical phase effect of the vortex on the interaction with atoms, and demonstrates that nondipole interactions induced by the XUV vortex are hardly observable in conventional gas-phase experiments.