用热解石墨晶体作色散和聚焦光学元件的激光等离子体源的超快X射线光谱仪(英文)

Pyrolytic graphite crystals as components for ultrafast X-ray spectroscopy using laser-based sources

由于激光等离子X射线源的光子通量显著低于同步辐射源的光子通量且射线为所有方向的各向同性辐射,所以,很需要具有大的集光立体角和高的积分反射率的光学元件,用热解石墨(PG)晶体作色散和聚焦元件可满足上述要求。由于PG晶体为嵌镶结构,所以可给出很高的积分反射率,而PG薄膜还可安装在任意形状的模具上构成任意形状的光学元件。此外,特殊形状的嵌镶聚焦使这些晶体甚至在弯曲的情况下,也可作为高分辨率X射线光学元件。基于上述元件特性,可以设计出有高集光效率的色散光学元件,用于激光等离子体源超快X射线光谱检测。文中描述了PG弯晶在一台改型的von HAMOS光谱仪中的应用,使用这台光谱仪,测量了飞秒激光器产生等离子体发射的X射线的光谱分布。讨论了产生的X射线在时间分辨扩展X射线吸收精细结构(EXAFS)分析中的应用。实验表明,通过优化晶体特性和光谱仪几何设置,可以实现对过渡金属K边的高分辨率EXAFS测量。

Because the photon flux of laser-based X-ray sources is considerably lower than that of synchrotron radiation sources and radiation is emitted isotropic in all directions, focusing optics with a large solid collecting angle and high integral reflectivity are required. For these requirements crystals of Pyrolytic Graphite （PG） are of particular interest for the use as dispersive and focusing elements. Due to the mosaic crystal structure, PG exhibits a very high integral reflectivity. Furthermore, thin PG films give the opportunity to realize crystal optics with arbitrary geometry by mounting them on a mold of any shape. Beyond that, mosaic focusing in specific geometry allows these crystals to be used as high resolution X-ray optics, even in bent geometry. All these properties allow the design of highly efficient dispersive collecting optics for uhrafast X-ray spectroscopy with laser plasma sources. In our contribution we describe the application of bent PG crystals in a spectrometer with a modified Von HAMOS geometry. Using this spectrometer, the spectral distribution of the X-ray radiation emitted from a fs laser produced plasma has been measured. The application of this radiation for timeresolved Extended X-ray Absorption Fine Structure （EXAFS） experiments is discussed. We show that, by optimizing both the crystal properties and the spectrometer geometry, a good spectral resolution sufficient for EXAFS measurements at the K-edges of transition metals can be achieved.