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.

Figure 1

Experimental setup used for the photoionization study of helium atoms using the XUV vortex. The XUV vortex generated by the APPLE-II undulator is introduced into the interaction point without any optical elements. Photoelectron angular distributions are obtained from the mapped electron image using the VMI spectrometer.

Figure 2

Simulation of the undulator radiation at the interaction point for the (a) first, (b) second, and (c) third harmonics. Left: two-dimensional flux density plots in units of photons per second per 0.1% bandwidth (b.w.) $\mathrm{per}\mathrm{m}{\mathrm{m}}^2$ for 30 eV photons. The intensity profiles along the horizontal and vertical center axes of the two-dimensional plots are attached. The first harmonic corresponds to a plane wave. The second and third harmonics correspond to XUV vortices carrying OAM of $l\hslash$ per photon with $l=1$ and 2, respectively. For the harmonic radiation, the phase of the electromagnetic field changes according to the azimuthal angle around a zero-intensity minimum corresponding to the phase singularity on the propagation axis. In the present study, the zero-intensity minimum associated with the phase singularity is blurred due to the finite emittance of the electron beam. In the measurements, undulator radiation is cut out by a 1-mm-diameter pinhole. The cut out areas are indicted by dotted circles. Right: partial photon flux obtained for the undulator radiation passing through the 1-mm-diameter pinhole.

Figure 3

Images and kinetic energy distributions of photoelectrons measured for the (a) first, (b) second, and (c) third harmonics from the helical undulator. The peak photon energies of the undulator radiation are 31, 32, and 32 eV for the first, second, and third harmonics, respectively. In the kinetic energy distributions, measurement and calculation results are plotted by solid red and dotted black curves, respectively.

Figure 4

Angular distributions of photoelectrons measured for the (a) first, (b) second, and (c) third harmonics from the helical undulator. The peak photon energies of the undulator radiation are 31, 32, and 32 eV for the first, second, and third harmonics, respectively. The solid blue curve in (a) represents a fit assuming electric dipole transitions. The dotted green curves in (b) and (c) show the angular dependence of the photoelectrons expected for nondipole transitions induced by the OAM carried by the XUV vortex. The solid blue curves in (b) and (c) represent fit results assuming both electric dipole and nondipole transitions.