Jark Werner
Elettra - Sincrotrone Trieste SCpA, SS 14 - km 163.5 in AREA Science Park, Basovizza, Trieste 34149, Italy.
J Synchrotron Radiat. 2020 Nov 1;27(Pt 6):1499-1509. doi: 10.1107/S1600577520011194. Epub 2020 Oct 14.
When the trajectory of an incident beam is oriented parallel to the grooves of a periodic grating structure the radiation beam is diffracted off-plane orthogonal to the plane of incidence. The diffraction efficiency in this condition is very high and in a grating with a sawtooth profile it can approach the reflection coefficient for a simple mirror, when the diffraction order of interest follows the direction for specular reflection at the flat part of the steps. When this concept is used in a plane grating in a monochromator for synchrotron radiation sources, the incident beam is almost always collimated in order to minimize any deterioration of the beam properties due to aberrations, which will be introduced in the diffraction process when an uncollimated beam is used. These aberrations are very severe when the groove density is constant. It will be shown that the effect of these aberrations can be corrected after the diffraction by the use of astigmatic focusing. The latter can be provided by a crossed mirror pair with different focal lengths in the corresponding orthogonal directions. Then a monochromator based on this concept can provide source size limited spectral resolution in an uncollimated incident beam. This is identical to the spectral resolution that can be provided by the same grating when operated at the same position in a collimated incident beam. The source size limited spectral resolution in this case corresponds to a high spectral resolving power of better than ΔE/E = 10 000 for photon energies around 300 eV in the soft X-ray range.
当入射光束的轨迹与周期性光栅结构的凹槽平行时，辐射光束会在与入射平面正交的平面外发生衍射。在这种情况下，衍射效率非常高，对于具有锯齿形轮廓的光栅，当感兴趣的衍射级遵循台阶平坦部分的镜面反射方向时，其衍射效率可接近简单镜子的反射系数。当这一概念应用于同步辐射源单色仪中的平面光栅时，为了将由于像差导致的光束特性恶化降至最低，入射光束几乎总是准直的，因为使用非准直光束时，在衍射过程中会引入像差。当凹槽密度恒定时，这些像差非常严重。结果表明，通过使用像散聚焦，可以在衍射后校正这些像差的影响。后者可由在相应正交方向上具有不同焦距的交叉镜对提供。这样，基于这一概念的单色仪可以在非准直入射光束中提供源尺寸受限的光谱分辨率。这与在准直入射光束中相同位置使用相同光栅时所能提供的光谱分辨率相同。在这种情况下，源尺寸受限的光谱分辨率对应于软X射线范围内光子能量约为300 eV时高于ΔE/E = 10 000的高光谱分辨能力。
