Photoelectron Angular Distributions for Two-photon Ionization of Helium   by Ultrashort Extreme Ultraviolet Free Electron Laser Pulses
  Phase-shift differences and amplitude ratios of the outgoing $s$ and $d$ continuum wave packets generated by two-photon ionization of helium atoms are determined from the photoelectron angular distributions obtained using velocity map imaging. Helium atoms are ionized with ultrashort extreme-ultraviolet free-electron laser pulses with a photon energy of 20.3, 21.3, 23.0, and 24.3 eV, produced by the SPring-8 Compact SASE Source test accelerator. The measured values of the phase-shift differences are distinct from scattering phase-shift differences when the photon energy is tuned to an excited level or Rydberg manifold. The difference stems from the competition between resonant and non-resonant paths in two-photon ionization by ultrashort pulses. Since the competition can be controlled in principle by the pulse shape, the present results illustrate a new way to tailor the continuum wave packet. 
R. Ma, K. Motomura, K. L. Ishikawa, S. Mondal, H. Fukuzawa, A. Yamada,   K. Ueda, K. Nagaya, S. Yase, Y. Mizoguchi, M. Yao, ...
physics.atom-ph