Hanle Effect for Lifetime Determinations in the Soft X-ray Regime

By exciting a series of 1s2 s 2 1 S 0 -> 1 snp 1 P 1 transitions in heliumlike 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 snp 1 P 1 levels, which find excellent agreement with atomic-structure calculations. We argue that dedicated soft x-ray measurements could yield lifetime data that are beyond current experimental reach and cannot yet be predicted with sufficient accuracy.