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

TL;DR

This study measures phase-shift differences and amplitude ratios in helium's two-photon ionization using ultrashort EUV pulses, revealing control over continuum wave packets through pulse shaping.

Contribution

It provides the first experimental determination of phase-shift differences in helium ionization with ultrashort EUV pulses, highlighting the influence of resonant and non-resonant pathways.

Findings

Phase-shift differences vary with photon energy near excited states.

Resonant and non-resonant pathways compete, affecting wave packet characteristics.

Pulse shaping can potentially control the ionization process.

Abstract

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.