Linear dichroism in few-photon ionization of laser-dressed helium

Severin Meister,Florian Trost,Harald Redlin,Xinhua Xie,Nicolas Douguet,Bastian Manschwetus,Hannes Lindenblatt,Thomas Pfeifer,Sven Augustin,Markus Braune,Aaron Bondy,Klaus Bartschat,Robert Moshammer,Kirsten Schnorr,Nora Schirmel

doi:10.1140/epjd/s10053-021-00218-0

Abstract

Ionization of laser-dressed atomic helium is investigated with focus on photoelectron angular distributions stemming from two-color multi-photon excited states. The experiment combines extreme ultraviolet (XUV) with infrared (IR) radiation, while the relative polarization and the temporal delay between the pulses can be varied. By means of an XUV photon energy scan over several electronvolts, we get access to excited states in the dressed atom exhibiting various binding energies, angular momenta, and magnetic quantum numbers. Furthermore, varying the relative polarization is employed as a handle to switch on and off the population of certain states that are only accessible by two-photon excitation. In this way, photoemission can be suppressed for specific XUV photon energies. Additionally, we investigate the dependence of the photoelectron angular distributions on the IR laser intensity. At our higher IR intensities, we start leaving the simple multi-photon ionization regime. The interpretation of the experimental results is supported by numerically solving the time-dependent Schrödinger equation in a single-active-electron approximation.Graphic abstract

Highlights

IntroductionScientists employ light to investigate matter to high precision, and as a tool to modify it in a well-controlled manner
The fundamental interaction of photons and matter is omnipresent in nature
We investigated photoelectron emission via excited states in laser-dressed atomic helium

Summary

Introduction

Scientists employ light to investigate matter to high precision, and as a tool to modify it in a well-controlled manner. Complementary transient absorption measurements employed an energetically broad XUV spectrum in combination with the IR driving laser and revealed light-induced states (LIS) in helium [7]. These LIS can be attributed to two-color multiphoton excitation, allowing transitions beyond the one-photon electricdipole selection rules. LIS were investigated in the context of transient absorption (see [8] for a review) with regard to their intensity dependence [9], quantum interference [10,11], and for molecules [12]. In the following we will refer to the basic helium eigenstates, but the notation obviously depends on the viewing angle

Methods

Results

Conclusion