IR-Assisted EUV Photoionization of Argon: Numerical Analysis of PADs and Fano’s Propensity Rule

In the late 1980s, experimenters observed a new phenomenon in the interaction between intense laser fields and gases, which is now known as high-harmonic generation (HHG).
The discovery of HHG paved the way for using infrared (IR) laser sources to produce femtosecond pulses in the extreme ultraviolet (EUV) and soft X-ray spectral ranges.
This nonlinear process also enables the study of two-color photoionization, where an atom is ionized by a combination of EUV and IR laser pulses rather than by a single pulse. In such experiments, the photoelectron energy spectrum reveals that the dominant contributions arise from processes in which an emitted electron, after absorbing an EUV photon, exchanges n infrared photons (n ≥ 1) with the dressing IR field via stimulated absorption or emission.
This results in the formation of photoelectron sidebands (SB±n) that are symmetrically distributed around the main harmonic peak, corresponding to the direct photoionization of the atom.

In this work, we perform numerical simulations of the nonresonant two-color photoionization of argon, using the combined action of an EUV pulse corresponding to the 13th harmonic of an infrared laser and the fundamental IR field itself.
The time-dependent Schrödinger equation (TDSE) is solved numerically within the single-active-electron (SAE) approximation, and the photoelectron wave packet is extracted using the window-operator technique.
This approach allows us to analyze both the photoelectron angular distributions (PADs) and the behavior predicted by the generalized Fano’s propensity rule in IR-assisted EUV photoionization of argon atoms.