Circular Dichroism in Multiphoton Ionization of Resonantly Excited Helium Ions near Channel Closing

TL;DR

This study investigates the circular dichroism in multiphoton ionization of excited helium ions near channel closing, revealing complex polarization-dependent effects and resonances through experimental and theoretical analysis.

Contribution

It provides the first detailed experimental and numerical analysis of circular dichroism in multiphoton ionization of resonantly excited helium ions near channel closing.

Findings

Complex variation of CD observed with laser intensity and polarization.

Resonance effects significantly influence the CD signal.

Numerical simulations agree with experimental results.

Abstract

The circular dichroism (CD) of photoelectrons generated by near-infrared (NIR) laser pulses using multiphoton ionization of excited He$^+$ ions in the 3p(m=+1) state. The ions were prepared by circularly polarized extreme ultraviolet (XUV) pulses. For circularly polarized NIR pulses co- and counter-rotating relative to the polarization of the XUV pulse, a complex variation of the CD is observed as a result of intensity- and polarization-dependent Freeman resonances, with and without additional dichroic AC-Stark shifts. The experimental results are compared with numerical solutions of the time-dependent Schr\"odinger equation to identify and interpret the pronounced variation of the experimentally observed CD.