Experimentally obtained anisotropy parameters β<sub>2</sub> and β<sub>4</sub>, and the extracted values of <em>W</em> and Δ

<p><b>Table 1.</b> Experimentally obtained anisotropy parameters β<sub>2</sub> and β<sub>4</sub>, and the extracted values of <em>W</em> and Δ.</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>