Wavenumber Calibration of CCD Detector Raman Spectrometers Controlled by a Sinus Arm Drive

Abstract

The calibration procedures needed for use of dispersive Raman spectrometers have been reviewed. Like other high‐precision spectrometers incorporating moving gratings, Raman spectrometers are subject to problems with wavenumber scale accuracy. Commercially available Raman spectrometers of types DILOR‐HORIBA LabRam and RENISHAW System 1000 have been examined for wavenumber scale stability, linearity, and reproducibility. For reliable use of the wavenumber data, daily calibration is a necessity. A procedure to examine the linearity of such mechanical drive systems is presented. A new finding was that the examined spectrometers give wavenumber calibration errors that were quite reproducible from day to day at a given temperature in the laboratory but depended markedly on the selected setting of the gratings. Knowledge of this linearity problem is essential for obtaining a reliable calibration. The most correct calibration was obtained by the use of certain “golden” settings of the sinus drive at a constant temperature of the laboratory. In this way, the examined spectrometers could be used with better precision, without daily calibration, provided the temperature of the room was constant or corrected for. A procedure for correction of these errors by the software is suggested.     

On the bremsstrahlung background correction to the high-energy Compton spectroscopy

methodology for bremsstrahlung (BS) background correction to extract a true Compton profile in high-energy Compton scattering experiments is presented. The BS background profiles for Hg, computed within the Born approximation, are estimated for different values of incident energy. It is seen for the first time that the BS background contribution in high-energy Compton profile experiments like those employing third generation synchrotron radiation sources comes out to be significant and non-linear. Further, it is found that the incorporation of BS correction in data reduction of such an experiment performed on Hg at 662 keV energy helps in reconciliation of theory and experiment.     

NO与C2H2的康普顿轮廓研究

基于第三代同步辐射光源, 在20 keV的入射X射线能量下测量了NO与C₂H₂分子的康普顿轮廓. 考虑到本次实验结果在p_z ≈ 0附近的统计精度达到了0.2%, 本文报道的NO和C₂H₂的康普顿轮廓可以作为严格检验理论的实验基准. 除此之外, 还分别采用HF方法及密度泛函方法选用不同的基组计算了NO 与C₂H₂康普顿轮廓. 通过对比实验结果与理论计算, 发现对于NO分子, 加入弥散函数基组理论计算结果与实验符合更好, 说明NO分子基态的电子分布较为弥散. 对于C₂H₂分子, HF方法理论计算的结果与实验符合较好.     

Compton scattering of 0.279 MeV gamma rays fromK-shell electrons of lead

Differential cross-sections for Compton scatterig of 0.279 MeV gamma rays fromK-shell electrons of lead are measured at scattering angles ranging from 30 to 150° to study the effect of binding on the scattering process. Measurements are made employing two Nal(Tl) scintillation spectrometers and a slow-fast coincidence set-up of 30 nsec resolving time. The experimental results are compared with the available theoretical data. The totalK-shell Compton scattering cross-section is 34% of the free electron Compton cross-section.     

Strong anisotropy in the low temperature Compton profiles of electron momentum distribution in α-Ga metal

Compton profiles of momentum distribution of conduction electrons in the orthorhombic phase of α-Ga metal at low temperature are calculated in the band model for the three crystallographic directions (100), (010), and (001). Unlike the results at room temperature, previously reported by Lengeler, Lasser and Mair, the present results show strong anisotropy in the Compton profiles with the momentum distribution along (001) direction being substantially different from the other two directions. While experimental data on Compton profiles at low temperatures are not available for comparison with theory, the resistivity data in α-Ga at low temperature strongly support this anisotropic behaviour. Besides, the electronic heat capacity constant γ available from both experiment and present calculation suggests that the conduction electron distribution at low temperature in the orthorhombic phase is markedly different from the free-electron-like-distribution at room temperature, thus lending additional support to anisotropic behaviour of Compton profiles. It would be nice to have Compton profiles data from experiment at low temperature for direct comparison with theory. It is hoped that the present work would stimulate enough interest in that direction.     

Hartree-Fock investigation of the electron momentum distribution in metallic beryllium

The wave function resulting from an accurate Hartree-Fock computation has been used to describe the electron momentum distribution in beryllium metal. Appreciable discrepancies are found with respect to a simple model that amounts to filling with plane waves the first plus second Brillouin zone. In comparing the calculated with the experimental directional Compton profiles, very close agreement is found in particular with a recent set of accurate -ray profiles. The differential Compton profiles are also semiquantitatively reproduced, better on average than with all previous calculations. Concerning the Fourier transforms of the Compton profiles, all the features of the experimental curves are accurately reproduced, in particular the position of the zero passages, which is a powerful test of the adequacy of a theoretical model to account for the metallic nature of the solid. The possible influence of the correlation correction is discussed, and it is shown that such a correction would substantially improve the overall agreement with the experiment.     

Geometrical efficiency,solid angle contribution and gradient for a triaxial system equipped with a fluorescent source and x‐ray tube: an analysis to enhance the Compton scattered photons

The total geometrical efficiency, solid angle contribution and gradient were estimated theoretically for a sample placed in a triaxial system equipped with a fluorescent source and x‐ray tube. The contribution of all of the above parameters were assessed at two different secondary target angles with distance. These values are useful for the triaxial system to measure the basic interaction cross‐sections excited by keV photons, to enhance the Compton scattered photons, estimation of monochromacy, solid angle correction and geometrical efficiency. Initially, the secondary target and sample are located at an angle of 45° and detected the scattered radiation from the sample to enhance the Compton scattered photons. By placing the sample at 90° in a triaxial system, the contribution of background is estimated. Comparing the data obtained in the two modes, the Compton photons are enhanced. An analysis of the above parameters and their importance for various potential applications are thoroughly discussed. Copyright © 2004 John Wiley & Sons, Ltd.     

The alignment of spectrometers and quantitative measurements in X-ray photoelectron spectroscopy

The alignment of the sample in X-ray photoelectron spectrometers is usually made to optimize the spectral intensities. There are two important classes of spectrometer: (i) those in which the analyser acceptance area is independent of the analysed electron kinetic energy; and (ii) those in which this area varies. Model experiments show how an example of a VG ESCALAB II conforms to class (i) whereas an example of an SSI X-probe is of class (ii) and shows an analyser acceptance area which depends approximately inversely on the emitted electron kinetic energy. This latter result means that the SSI X-probe spectrometer must be aligned for the electrons of the highest kinetic energy (smallest analyser acceptance area). A misalignment of 0.1 mm in the sample height can cause a 10% change in the relative intensities between 0 and 1000 eV binding energies. This dependence of the analyser acceptance area with energy is an effect likely to be common in the advanced electron optical systems of modern electron spectrometers and should be understood in order to use such spectrometers effectively. Such dependencies should be determined by analysts for their own instruments in the operating mode that is used for conducting work in which the repeatability of intensity measurements is important.     

Density-functional theory of the Compton profile anisotropy of copper metal

Band structure calculations of the Compton profiles of copper are compared with the experiment in the light of a rigorous interpretation of momentum densities in the Hohenberg-Kohn-Sham ground state density-functional formalism. There are strong evidences that the significant discrepancies for the Compton profile anisotropies do not originate from inaccurate solutions of the Kohn-Sham self-consistent equations, but from a non-local momentum density correlation correction functional.     

Anisotropy in the compton profile of copper

Directional Compton profiles of high statistical accuracy are measured by means of a 412 keV gamma-ray Compton spectrometer. The experimental anisotropies are in very satisfactory agreement with earlier measurements and there is good qualitative agreement between the experimental data and a recent band structure calculation. Quantitatively, however, the experimental anisotropy is significantly smaller than predicted by theory.A simple model calculation based on the Seitz approximation, in which higher order Fermi surface volumes all of spherical shape are taken into account in obtaining the momentum density, demonstrates that the major contribution to the Compton profile anisotropy in copper is due to the hybridization which has mixedd-electrons and nearly-free electrons in the top band. Any fine structure in the theoretical anisotropy due to the detailed shape of the Fermi surface cannot be resolved in the present experiment.The potential of analysing the electronic structure of transition metals in terms ofB(r), the Fourier transform of the momentum density, is discussed in detail. The major contribution to the anisotropy arises from localised peaks inB(r) centered on the sites of the translational lattice. Within the Seitz approximation it could be shown that these secondary maxima are caused by the presence of localisedd-electrons in the highest partly occupied band. In conclusion we anticipate that a proper treatment of electron correlation would produce a marked quantitative improvement in the agreement between the present experimental data and the Compton profiles obtained from current band structure calculations.     

A Cauchois-type X-ray spectrometer for momentum density studies on heavy-element materials

This article describes the new spectrometer setup for high-resolution Compton scattering experiments at BL08W, SPring-8. It employs an X-ray image intensifier (X-II) system as a position sensitive detector (PSD). The installation of the X-II system has improved the data-acquisition efficiency by 20 times compared to the previous quasi-PSD system. Compton profiles of CeRh₃B₂ have been measured with sufficient statistical accuracy, which indicates that the new setup can measure Compton profiles of virtually all materials within a reasonable beamtime.     

Performance of 20 Ci137Cs γ-ray Compton spectrometer for the study of momentum densities

In this paper, we present the design and construction of a 20 Ci γ-ray Compton spectrometer that employs a¹³⁷ Cs source with a strong line at 661.65 keV. The total resolution of the spectrometer in momentum scale is 0.40 a.u., which is much better than the conventional²⁴¹Am Compton spectrometers. The in-house¹³⁷Cs spectrometer is very useful for the measurement of momentum densities of heavy materials. The performance of the machine is assessed using aluminum, terbium and mercury samples and the experimental data from comparable apparatus.     

偏振型干涉成像光谱仪谱线位置定标方法的研究

论述了偏振型干涉成像光谱仪的工作原理, 针对复原光谱谱线位置漂移问题, 提出了原理修正和数据处理两种具有代表性的实验室谱线位置定标方法, 给出了定标结果及对比分析. 原理修正方法从干涉型成像光谱仪的参数选择着手, 分析了产生谱线位置漂移的原因, 针对复原谱线位置随行变化的问题, 给出了修正方案, 使得谱线位置精度明显提高; 对于给定的四组激光器标准波长, 谱线位置均方根误差由定标前的28.3914下降至5.5371, 该方法对干涉型成像光谱仪具有普适性, 且其定标参数对分析仪器指标提供了便利. 数据处理方法弥补了原理修正定标存在的数据量大、短波定标效果弱等弊端, 谱线位置均方根误差下降至0.9178, 该方法实施简单, 对不同的输入波长, 所取不同行的数据用统一的表达式进行修正. 该方法化繁为简、间接定标的思想具有一定的借鉴价值. 该研究为偏振型干涉成像光谱仪的设计、研制、调试和工程化提供了重要的理论依据和实践指导.     

Compton profiles of heavy atoms by inelastic proton-electron scattering

So-called ion Compton profiles can be obtained if the recoiling electron after an inelastic ion-electron encounter is observed and energy analyzed. Electron recoil spectra induced by 21 MeV protons passing through thin Ag and Au foils are measured. It is demonstrated that the method is accurate enough to extract valence Compton profiles of Ag and Au from the data. The advantages and disadvantages compared to inelastic - or electron-electron scattering are discussed. It turns out that this new method for the measurement of Compton profiles is especially suitable for heavy elements and very thin or small targets (e.g. clusters).     

Directional compton profile studies of GaP

59.54 keV gamma ray Compton profiles (CP) of GaP along (100) and (1 1 1) directions are reported. The measured anisotropy has qualitatively similar features as given by the recent pseudopotential calculations for other III–V compound semiconductors. The average valence profile is found to be more peaked compared to that of the pure covalent semiconductors Si and Ge. This suggests that inonicity has the effect of making the valence-electron wave functions of compound semiconductors more diffused. Theoretical calculations on the directional Compton profiles of GaP will be of great use for an exact comparision with the present data, which will help in detailed understanding of the electronic structure of GaP.     

Monte-Carlo simulation on Compton double scattering of polarized X-rays

A Monte-Carlo simulation of energy spectra of Compton double scattered X-rays has been made as functions of the incident energy of elliptically polarized X-rays and the atomic number of samples having different shapes and volumes. This simulation is based on the previous one made by one of the authors (N.S.), in which completely circularly polarized γ-rays were only assumed, and is improved to meet the multiple scattering correction on experimental Compton profiles measured with linearly or elliptically polarized synchrotron radiations. The results are compared with two experiments in reasonable agreement.     

Molecular Compton profiles from SCF-MO wavefunctions

Compton profiles are derived from ab initio SCF-MO wavefunctions calculated for cyclopropane, propene, aziridine, ethanimine, ethenamine, oxirane, acetaldehyde and formamide using Dunning's gaussian orbital basis. Good agreement with the experimental profile is found for formamide. The differences between the profiles for the cyclic and acyclic isomers are only slightly larger than the experimental accuracy that can presently be achieved. The cyclic compounds have slightly broader profiles and there are no significant differences arising from strain. Comparison with profiles derived from localized orbital contributions indicate that the localized orbital approach is essentially valid for these systems.     

The comparative analytical performance of the Beagle 2 X‐ray Spectrometer for in situ geochemical analysis on Mars

The Beagle 2 X‐ray Spectrometer (B2 XRS) instrument was part of the Beagle 2 Mars lander payload and intended to perform in situ geochemical analyses of geological materials on Mars. The analytical performance of a spare version of the B2 XRS was compared with (1) a portable X‐ray fluorescence (PXRF) spectrometer designed to perform terrestrial fieldwork and (2) a laboratory‐based wavelength‐dispersive (WD‐XRF) system used to produce high quality geochemical data. The criteria used to assess the performance were based on fitting precision, accuracy and detection limit, determined from the analysis of international geochemical reference materials. The fitting precision of the B2 XRS and PXRF was found to be in agreement with the Horwitz function (a benchmark relating the analysed concentration of an analyte to its uncertainty) over 4 orders of magnitude of concentration range from 10^?1 to 10^?5 g/g. The PXRF generally had a better fitting precision than the B2 XRS because of its better resolution. In order of improving accuracy, the spectrometers generally are ranked B2 XRS, PXRF and WD‐XRF for various major and trace elements. A limiting factor in the accuracy of the B2 XRS was the application of the algorithm used for its quantitative analysis. The detection limits for the spectrometers ranked in the same order as the accuracy as a result of improving signal‐to‐noise ratio (SNR) of elemental peaks, which is a direct consequence of improving resolution between these spectrometers. Overall, the B2 XRS was found to have a favourable analytical performance compared to the benchmark spectrometers, despite having met considerable design constraints and qualification tests as a planetary instrument. Copyright © 2009 John Wiley & Sons, Ltd.