Two-dimensional projection (left) of the momentum distribution of the outgoing photoelectrons from the TPI of helium (He) atoms at 20.3 eV photon energy, and the corresponding slice (right) through the retrieved 3D photoelectron momentum distribution obtained from an Abel inversion

<p><strong>Figure 1.</strong> Two-dimensional projection (left) of the momentum distribution of the outgoing photoelectrons from the TPI of helium (He) atoms at 20.3 eV photon energy, and the corresponding slice (right) through the retrieved 3D photoelectron momentum distribution obtained from an Abel inversion.</p> <p><strong>Abstract</strong></p> <p>The two-photon ionization of helium atoms by ultrashort extreme-ultraviolet free-electron laser pulses, produced by the SPring-8 Compact SASE Source test accelerator, was investigated at photon energies of 20.3, 21.3, 23.0 and 24.3 eV. The angular distribution of photoelectrons generated by two-photon ionization is obtained using a velocity map imaging spectrometer. The phase-shift differences and amplitude ratios of the outgoing s and d continuum wave packets are extracted from the photoelectron angular distributions. The obtained values of the phase-shift differences are distinct from scattering phase-shift differences when the photon energy is tuned to a resonance with an excited level or Rydberg manifold. The difference stems from the co-presence of resonant and non-resonant path contributions in the two-photon ionization by femtosecond pulses. Since the relative contribution of both paths can be controlled in principle by the pulse shape, these results illustrate a new way to tailor the continuum wave packet.</p>