Theoretical study of pulse delay effects in the photoelectron angular distribution of near-threshold EUV+IR two-photon ionization of atoms

TL;DR

This theoretical study investigates how the delay between EUV and IR laser pulses influences the angular distribution of photoelectrons in two-photon ionization of hydrogen and noble gases, revealing delay-dependent anisotropy parameters.

Contribution

The paper provides a detailed theoretical analysis of pulse delay effects on photoelectron angular distributions using perturbation theory and Schrödinger equation solutions, highlighting the delay dependence of anisotropy parameters.

Findings

Delay affects PAD anisotropy parameters for most atoms studied.

Resonant and nonresonant ionization paths interplay varies with delay.

Pulse delay has a stronger effect on p-shell than s-shell ionization.

Abstract

We theoretically study the photoelectron angular distributions (PADs) from two-color two-photon near-threshold ionization of hydrogen and noble gas (He, Ne, and Ar) atoms by a combined action of femtosecond extreme ultraviolet (EUV) and near-infrared (IR) laser pulses. Using the second-order time-dependent perturbation theory, we clarify how the two-photon ionization process depends on EUV-IR pulse delay and how it is connected to the interplay between resonant and nonresonant ionization paths. Furthermore, by solving the time-dependent Schr\"odinger equation, we calculate the anisotropy parameters $\beta_2$ and $\beta_4$ as well as the amplitude ratio and relative phase between partial waves characterizing the PADs. We show that in general these parameters notably depend on the time delay between the EUV and IR pulses, except for He. This dependence is related to the varying relative role of resonant and nonresonant paths of photoionization. Our numerical results for H, He, Ne, and Ar show that the pulse-delay effect is more pronounced for $p$-shell ionization than for $s$-shell ionization.