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.

中文翻译：

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由种子自由电子激光器的椭圆极化脉冲实现的磁性过渡金属 Tedge 的圆二色性实验


我们已经从种子自由电子激光器 （FEL） 在 700-800 eV 的光谱范围内产生了椭圆偏振飞秒脉冲，并在 Fe L3 边缘进行了基于磁圆二色性的演示实验。这是通过利用基波 FEL 波长 λ 的三次谐波产生的辐射来实现的。平面辐射器沿起伏器轴线发射线性极化谐波，而螺旋起伏器产生圆极化的离轴谐波辐射。然而，离轴圆偏振光的特点是通量相当低，因此，它不适用于大多数依赖圆二色性的实验。在这里，我们证明了在 λ/3 处具有大量圆极化的轴上辐射可以使用可变极化（APPLE 型）辐射器产生，并在混合极化模式下进行微调。这些实验证明了所采用的构型在磁二色性研究中的潜力，是在的里雅斯特（意大利）的 FERMI FEL 设施进行的。这些结果为与磁性研究最相关的过渡金属（Fe、Co、Ni）的 2p-3d 共振的实验铺平了道路，并可以设想研究，例如，具有不同核孔寿命的光谱研究，涵盖 M （50-70 eV） 和 L （600-900 eV） 边缘，在同一种子 FEL 源。