Enhanced One-Color-Two-Photon Resonant Ionization in Highly Charged Ions by Fine-Structure Effects

TL;DR

This paper demonstrates how relativistic fine-structure effects enable highly efficient two-photon ionization in highly charged ions using X-ray free-electron lasers, revealing complex ionization pathways and potential for precision X-ray metrology.

Contribution

It shows that fine-structure effects significantly enhance two-photon ionization efficiency in highly charged ions, providing new insights into nonlinear X-ray interactions.

Findings

Resonant two-photon ionization efficiency increased by over two orders of magnitude.

Fine-structure effects shift electronic energies and enable resonant absorption.

Dominance of doubly-resonant channels in nonlinear X-ray interactions.

Abstract

Ultraintense pulses from X-ray free-electron lasers can drive, within femtoseconds, multiple processes in the inner shells of atoms and molecules in all phases of matter. The ensuing complex ionization pathways of outer-shell electrons from the neutral to the final highly charged states make a comparison with theory enormously difficult. We resolve these pathways by preparing highly charged ions in an electron beam ion trap before exposing them to the pulsed radiation. This reveals how relativistic fine-structure effects shift electronic energies, largely compensate the core-screening potential, and enable the consecutive, resonant absorption of two quasi-monochromatic X-ray photons that would generally be unfeasible. This doubly-resonant channel enhances the efficiency of two-photon ionization by more than two orders of magnitude, dominating in this regime the nonlinear interaction of light and matter with possible application for future precision X-ray metrology.