In 1924, physicist Wilhelm Hanle gave a semiclassical explanation for the observed depolarization of fluorescence light emitted from atom in a magnetic field. The “Hanle effect” was later described within quantum theory by George Breit in 1933. Since the degree of polarization depends on the lifetime of the excited atomic states, the Hanle effect has been used as a practical tool for lifetime determinations of optical transitions. Traditionally, in Hanle effect experiments a particular excited state of interest is considered while the magnetic field is varied.
In a recent study, researchers lead by the Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg demonstrated a “soft x-ray Hanle effect” for several excited states in a fixed magnetic field. They used linearly polarized monochromatic soft X rays at the Elettra facility in Trieste (Italy) to excite a series of transitions in heliumlike nitrogen ions produced in a compact electron beam ion source (EBIT) developed at MPIK. As a result, they observed a systematic change in the angular distribution of the fluorescence for the series of excited states. In particular, the ratio of emission in directions parallel and perpendicular to the polarization of incident radiation was found to increase with higher the principal quantum number n – as a manifestation of the Hanle effect.
By comparing experimental data with theoretical predictions, the researchers determined lifetimes in a range from hundreds of femtoseconds to tens of picoseconds. The results were found to be in excellent agreement with atomic-structure calculations for theoretically well-understood helium-like ions. This new method based on the Hanle effect opens a general access to lifetimes of excited levels in highly charged ions in the soft x-ray regime. As a next step, it will be applied to lithium-like and other isoelectronic sequences in highly charged ions which still challenge theory. Experimental results for such transitions are of great importance in the analysis of X-ray space observatory data as well as for applications in magnetic field and polarization studies of hot plasmas.
Original publication:
Hanle Effect for Lifetime Determinations in the Soft X-Ray Regime
Moto Togawa, Jan Richter, Chintan Shah, Marc Botz, Joshua Nenninger, Jonas Danisch, Joschka Goes, Steffen Kühn, Pedro Amaro, Awad Mohamed, Yuki Amano, Stefano Orlando, Roberta Totani, Monica de Simone, Stephan Fritzsche, Thomas Pfeifer, Marcello Coreno, Andrey Surzhykov and José R. Crespo López-Urrutia
Physical Review Letters 133, 163202 (2024). DOI: 10.1103/PhysRevLett.133.163202
Weblinks:
Group "Highly charged ion dynamics" at MPIK
