Competition of resonant and nonresonant paths in resonance-enhanced two-photon single ionization of He by an ultrashort extreme-ultraviolet pulse

TL;DR

This paper investigates how pulse width influences the photoelectron angular distribution in resonance-enhanced two-photon ionization of helium, revealing the interplay between resonant and nonresonant ionization paths and their effect on phase differences.

Contribution

It provides a detailed theoretical analysis of the pulse-width dependence of PAD and phase differences in helium ionization, highlighting the role of resonant and nonresonant pathways.

Findings

PAD and phase difference vary with pulse width when resonant with an excited level.

PAD remains relatively unchanged with pulse width when exciting a Rydberg manifold.

Short pulses significantly affect the phase and angular distribution in the ionization process.

Abstract

We theoretically study the pulse-width dependence of the photoelectron angular distribution (PAD) from the resonance-enhanced two-photon single ionization of He by femtosecond ($\lesssim 20$ fs) extreme-ultraviolet pulses, based on the time-dependent perturbation theory and simulations with the full time-dependent Schr\"odinger equation. In particular, we focus on the competition between resonant and nonresonant ionization paths, which leads to the relative phase $\delta$ between the $S$ and $D$ wave packets distinct from the corresponding scattering phase shift difference. When the spectrally broadened pulse is resonant with an excited level, the competition varies with pulse width, and, therefore, $\delta$ and the PAD also change with it. On the other hand, when the Rydberg manifold is excited, $\delta$ and the PAD do not much vary with the pulse width, except for the very short pulse regime.