Circular dichroism experiments at the L edge of magnetic transition metals enabled by elliptically polarized pulses from a seeded free-electron laser

We have generated elliptically polarized femtosecond pulses from a seeded free-electron laser (FEL) in the spectral range 700–800 eV and conducted demonstrative experiments based on magnetic circular dichroism, at the Fe 𝐿3 edge. This was achieved by making use of the radiation produced at the third harmonic of the fundamental FEL wavelength λ. A planar radiator emits linearly polarized harmonics along the undulator axis, while a helical undulator produces off-axis harmonic radiation with circular polarization. However, off-axis circularly polarized light is characterized by a rather low flux and, as a result, it is unsuitable for most experiments that rely on circular dichroism. Here, we demonstrate that on-axis radiation with a substantial degree of circular polarization at λ/3 can be generated using a variable-polarization (APPLE-type) radiator, finely tuned in a hybrid-polarization mode. The experiments, which demonstrate the potential of the adopted configuration for magnetic dichroism studies, were carried out at the FERMI FEL facility in Trieste (Italy). These results pave the way to experiments at the 2⁢𝑝−3⁢𝑑 resonances of the transition metals most relevant for magnetic studies (Fe, Co, Ni) and make it possible to envisage investigations, e.g., spectroscopic studies with different core-hole lifetimes, covering both the 𝑀 (50–70 eV) and 𝐿 (600–900 eV) edges, at the same seeded FEL source.