Hanle Effect for Lifetime Determinations in the Soft X-Ray Regime.

By exciting a series of 1s^{2} ^{1}S_{0}→1snp^{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.