Dichroic Electron Emission Patterns from Oriented Helium Ions

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 $10^{12}W/c{\mathrm{m}}^2$ to $10^{13}W/c{\mathrm{m}}^2$. 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.