Nonsequential double ionization of helium in IR+XUV two-color laser fields II: Collision-excitation ionization process

TL;DR

This paper studies the collision-excitation-ionization mechanism in nonsequential double ionization of helium under IR+XUV laser fields, revealing how different photon energies and collision dynamics influence the process and resulting spectra.

Contribution

It extends previous work to analyze the CEI mechanism in NSDI, highlighting the dominant role of XUV photon absorption and interference pattern reconstruction in two-color laser fields.

Findings

CEI mechanism dominates when XUV photon energy is below He+ ionization threshold.

Complex interference patterns observed in momentum spectra.

Interference patterns can be reconstructed from ATI spectra.

Abstract

The collision-ionization mechanism of nonsequential double ionization (NSDI) process in IR+XUV two-color laser fields [\PRA \textbf{93}, 043417 (2016)] has been investigated by us recently. Here we extend this work to study the collision-excitation-ionization (CEI) mechanism of NSDI processes in the two-color laser fields with different laser conditions. It is found that the CEI mechanism makes a dominant contribution to the NSDI as the XUV photon energy is smaller than the ionization threshold of the He$^+$ ion, and the momentum spectrum shows complex interference patterns and symmetrical structures. By channel analysis, we find that, as the energy carried by the recollision electron is not enough to excite the bound electron, the bound electron will absorb XUV photons during their collision, as a result, both forward and backward collisions make a comparable contributions to the NSDI processes. However, it is found that, as the energy carried by the recollision electron is large enough to excite the bound electron, the bound electron does not absorb any XUV photon and it is excited only by sharing the energy carried by the recollsion electron, hence the forward collision plays a dominant role on the NSDI processes. Moreover, we find that the interference patterns of the NSDI spectra can be reconstructed by the spectra of two above-threshold ionization (ATI) processes, which may be used to analyze the structure of the two separate ATI spectra by NSDI processes.