Design of a compact spectrometer under grazing incidence conditions for near-edge X-ray absorption spectroscopy
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摘要:近边X射线吸收精细结构（NEXAFS）谱包含了吸收原子的局域结构信息，由于其适用范围广，灵敏度高，已经成为研究物质结构的重要手段之一。为了研究有机物的碳1s NEXAFS谱，本文基于气体 等离子体X射线光源，采用具有平场特性的凹面变线距光栅作为分光元件，面阵CCD作为光谱探测器，设计了一台小型掠入射式近边X射线吸收谱仪。通过优化光栅和CCD的装配方案，得到了入射角88.6°的装配参数。利用光线追迹法分析了谱仪的分辨率，该谱仪工作波段2~5 nm，在4.4 nm处分辨率可达666。通过分析各结构参量误差对谱线半高宽的影响发现，半高宽对入射角的误差最为敏感，优化的装配方案可以实现入射角的高精度调节。利用氮气等离子体光谱测试了光谱仪的性能，结果显示分辨率达到设计指标。Abstract:Near edge X-ray absorption fine structure(NEXAFS) spectrum contains the local structure information of the absorbing atoms. Due to its wide range of applications and high sensitivity, it has become an important method to study the structure of materials. In order to study the NEXAFS spectrum of carbon in organic substances, a design of a compact spectrometer under grazing incidence conditions is presented, based on a laser-produced plasma source using a gas target. An aberration corrected flat-field grating and a CCD camera are used to detect the spectrum. To achieve the high precision adjustment of the incident angle of the grating, an optimized scheme where the incident angle is 88.6° has been given. Using ray tracing, the resolution of the spectrometer is analyzed. It is about 666 at 4.4 nm in the wavelength range of 2-5 nm. From the influence of each parameter error on the image width, it is found that the error of the incident angle is the most sensitive, and using the optimized assembly scheme is able to achieve the high accuracy adjustment of the incident angle. The performance of the spectrometer has been tested using the nitrogen plasma spectrum, and the results show that the resolution reaches the design index.
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图 1掠入射式X射线吸收谱仪示意图
Figure 1.Sketch of the designed X-ray absorption spectrometer under grazing incidence conditions
图 3变栅距凹面光栅原理图
Figure 3.Schematic diagram of the aberration corrected flat-field grating
图 4不同入射角对应的聚焦曲线，红线表示聚焦曲线，黑色虚线表示其拟合直线，绿线表示不同波长
Figure 4.Focusing curves and fitting straight lines at different incidence angles. The red lines represent the focus curves, the black dashed lines are straight fitting lines and the green lines are different wavelengths
图 5光栅的不同使用结构
Figure 5.Structures of the grating (a)the conventional scheme, (b)the optimized scheme
图 64.4 nm波长的光线追迹结果
Figure 6.The result of ray tracing at 4.4 nm (a)Spot diagram, (b)Histogram, (c)Gauss fitting curve
图 7各参数(r、D、θ、α)的误差对4.4 nm单色谱线的半高宽的影响
Figure 7.Influence of the error of each parameter on the FWHM at 4.4 nm (a)r, (b)D, (c)θ, (d)α
表 1优化的装配方案参数
Table 1.Parameters of the optimized installation scheme
Parameters α/(°) r/mm D/mm θ/(°) Value 88.6 270 234.0 91.4 
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表 2掠入射式近边X射线吸收谱仪设计参数
Table 2.Design parameters of the Near-edge X-ray absorption spectrometer under grazing incidence conditions
Parameters Value Wavelength range/nm 2~5 Resolution 666@4.4 nm φ/(°) 2 Width of entrance slitS1/μm 100 n0/mm-1 2 400 Incidence angleα/(°) 88.6 Incidence distancer/mm 270 θ/(°) 91.4 D/mm 234.0 Pixel size(W×H:μm2) 13.5×13.5 Active pixels 1 024×1 024 下载:
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表 3波长标定结果
Table 3.Results of wavelength calibration
Pixel positions 1 631 777 782 821 841 1 024 Actural wavelength/nm 2.878 7 2.489 8 2.478 1 2.377 4 2.327 7 Calibration wavelength/nm 5.025 2.878 1 2.490 2 2.477 5 2.378 2 2.328 1 2.008 3 Error/nm -0.000 6 0.000 4 -0.000 5 0.000 8 0.000 4 下载:
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表 4光学系统参数设计值与实际值比较
Table 4.Comparison of the design value with the actual value of optical system parameters
Parameters α/(°) θ/(°) D0/mm x0/mm Design value 88.6 91.4 234 23.74 Actual value 88.634 8 91.389 9 234.011 3 23.682 6 下载:
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