(a) Amplitude ratio <em>W</em> and (b) phase-shift difference (relative phase) Δ extracted from experimental PADs (red diamonds) and calculated from wavefunctions obtained by TDSE simulations (black bullets)

<p><strong>Figure 3.</strong> (a) Amplitude ratio <em>W</em> and (b) phase-shift difference (relative phase) Δ extracted from experimental PADs (red diamonds) and calculated from wavefunctions obtained by TDSE simulations (black bullets). Theoretical scattering phase-shift difference Δ<sub>sc</sub> [<a href="http://iopscience.iop.org/0953-4075/46/16/164018/article#jpb461347bib49" target="_blank">49</a>] is represented by a blue solid line in panel (b).</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>