Hanle effect for lifetime determinations in the soft X-ray regime

TL;DR

This paper demonstrates a novel application of the Hanle effect in the soft X-ray regime to determine lifetimes of excited states in helium-like nitrogen ions, showing good agreement with theoretical calculations.

Contribution

It introduces a 'soft X-ray Hanle effect' method for lifetime measurements across multiple excited states in a static magnetic field, expanding beyond traditional approaches.

Findings

Lifetimes of 1snp 1P1 levels range from hundreds of femtoseconds to tens of picoseconds.

The experimental results agree well with atomic-structure calculations.

Soft X-ray measurements can potentially access lifetimes beyond current experimental capabilities.

Abstract

By exciting a series of $1\mathrm{s}^{2}\, ^{1}\mathrm{S}_{0} \to 1\mathrm{s}n\mathrm{p}\, ^{1}\mathrm{P}_{1}$ transitions in helium-like nitrogen ions with linearly polarized monochromatic soft X-rays at the Elettra facility, we found a change in the angular distribution of the fluorescence sensitive to the principal quantum number $n$. In particular it is observed that the ratio of emission in directions parallel and perpendicular to the polarization of incident radiation increases with higher $n$. We find this $n$-dependence to be a manifestation of the Hanle effect, which served as a practical tool for lifetime determinations of optical transitions since its discovery in 1924. In contrast to traditional Hanle effect experiments, in which one varies the magnetic field and considers a particular excited state, we demonstrate a 'soft X-ray Hanle effect' which arises in a static magnetic field but for a series of excited states. By comparing experimental data with theoretical predictions, we were able to determine lifetimes ranging from hundreds of femtoseconds to tens of picoseconds of the $1\mathrm{s}n\mathrm{p}\, ^{1}\mathrm{P}_{1}$ levels, which find excellent agreement with atomic-structure calculations. We argue that dedicated soft X-ray measurements could yield lifetime data that is beyond current experimental reach and cannot yet be predicted with sufficient accuracy.