Dichroic Electron Emission Patterns from Oriented Helium Ions

TL;DR

This study combines experimental and theoretical approaches to investigate dichroic electron emission patterns from oriented helium ions exposed to polarized laser pulses, revealing intensity-dependent angular distributions and multiple ionization pathways.

Contribution

It introduces a novel two-color photo-ionization scheme with polarization control to analyze dichroism in helium ion electron emission patterns, combining experimental data with theoretical modeling.

Findings

Electron emission exhibits strong dichroism depending on NIR intensity.

Distinct ionization pathways are identified for co-rotating and counter-rotating polarization cases.

Relative strength and phase of pathways are determined for counter-rotating case.

Abstract

We report a joint experimental and theoretical study using a combination of polarization-controlled free-electron-laser (FEL) and near-infra\-red (NIR) pulses in a synchronized two-color photo\-ionization scheme. Excited He$^+$ ions, created by extreme ultraviolet (XUV) circularly polarized radiation from the XUV-FEL FERMI in the oriented $3p\, (m\!=\!+1)$ state, are exposed to circularly polarized 784-nm NIR radiation with peak intensities from $10^{12}\,\rm W/cm^2$ to $\rm 10^{13}\,W/cm^2$. The angular distribution of the ejected electrons exhibit a strong dichroism depending on the NIR intensity. While the co-rotating case is defined by a single path, for the counter-rotating case, there are two dominant pathways whose relative strength and phase difference are determined.