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Phase-shift differences and amplitude ratios of the outgoing $s$ and $d$continuum wave packets generated by two-photon ionization of helium atoms aredetermined from the photoelectron angular distributions obtained using velocitymap imaging. Helium atoms are ionized with ultrashort extreme-ultravioletfree-electron laser pulses with a photon energy of 20.3, 21.3, 23.0, and 24.3eV, produced by the SPring-8 Compact SASE Source test accelerator. The measuredvalues of the phase-shift differences are distinct from scattering phase-shiftdifferences when the photon energy is tuned to an excited level or Rydbergmanifold. The difference stems from the competition between resonant andnon-resonant paths in two-photon ionization by ultrashort pulses. Since thecompetition can be controlled in principle by the pulse shape, the presentresults illustrate a new way to tailor the continuum wave packet.

Photoelectron Angular Distributions for Two-photon Ionization of Helium by Ultrashort Extreme Ultraviolet Free Electron Laser Pulses