The Marple algorithm for the autoregressive spectral estimates of the SMMW fourier transform spectroscopy data

The Marple algorthm for the autoregressive spectral estimates has been applied to the SMMW Fourier transform spectrum analysis. The experimental results have shown that this method yields AR spectra with three times higher resolution than the FFT method does. The improvements obtained from the Marple algorithm over the maximum entropy algorithm include higher resolution, less bias in the spectral peak frequency estimation and absence of observed spectral line splitting. The effects of the structure of the spectral lines and the noise on the resolution are discussed.Projects supported by National Natural Science Foundation of China     

FTIR单边干涉图相位校正技术的研究与改进

基于傅里叶变换红外光谱仪干涉图的单边过零采样和非对称性,对相位校正技术进行研究和改进。针对Mertz乘积法对切趾函数的匹配性要求高,干涉图非对称切趾旁瓣抑制效果差、光谱分辨率低的缺点,综合Mertz法和Forman法的特点提出一种改进的相位校正方法。采用对称化方法将单边干涉图变换为双边干涉图、对称切趾、傅里叶变换重建功率谱,并采用对称化的双边干涉图傅里叶变换求取高分辨率的相位谱,提高相位校正的精度。通过仿真分析了非对称窗函数较对称窗函数有比较宽的主瓣宽度、高的旁瓣幅值,分辨率降低;实验验证该改进的相位校正方法功率谱误差小、计算量少,且光谱分辨率比较高,达到2cm-1。     

Laser and Fourier transform emission spectroscopy of TaCl

The emission spectrum of TaCl has been recorded at high resolution in the 3000-35 000 cm⁻¹ region using a Fourier transform spectrometer. The bands were observed by microwave excitation of a mixture of TaCl₅ vapor and 3.0 Torr of He. Several TaCl bands have also been recorded using the laser ablation/molecular beam source at the University of New Brunswick. A rotational analysis of a number of bands has been obtained and the majority of the stronger bands have been classified into three groups with different lower state spectroscopic constants. The three lower states have been identified as having Ω″ = 0⁺, Ω″ = 2, and (tentatively) Ω″ = 3. The Ω″ = 0⁺ and Ω″ = 2 states are very close in energy and one of these two states is the ground state of TaCl.     

Emission spectroscopy of a new Δ-1Δ system of VO

High-resolution spectra of VO have been reinvestigated in the 12 000-31 000 cm⁻¹ region. VO was produced in a vanadium hollow cathode lamp by discharging 1.5 Torr of Ar and the spectra were recorded using a Fourier transform spectrometer. The oxygen needed to produce VO was present in the system as an impurity. Three new bands observed in the 21 000-22 100 cm⁻¹ region have been attributed to a new ²Δ-1²Δ electronic transition of VO. Two bands, with origins near 21 044 and 22 038 cm⁻¹, have been assigned as the 0-1 and 0-0 bands of the ²Δ_3/2-1²Δ_3/2 sub-band while a weak band with an origin near 21 975 cm⁻¹ has been assigned as the 0-0 band of the corresponding ²Δ_5/2-1²Δ_5/2 sub-band. A rotational analysis of these sub-bands has been obtained and spectroscopic constants have been extracted. The 1²Δ state is known from the previous analyses of the doublet transitions of VO in the near infrared. The present observation has allowed the determination of the vibrational interval ΔG_1/2 and the equilibrium rotational constants for the 1²Δ_3/2 state.     

Fourier transform emission spectroscopy and ab initio calculations on NbCl

The emission spectrum of NbCl has been recorded in the 3000-20 000 cm⁻¹ region using a Fourier transform spectrometer. The bands were observed by microwave excitation of a mixture of NbCl₅ vapor and He. Two groups of bands observed in the 6500-7000 cm⁻¹ and 9800-11 000 cm⁻¹ regions have been assigned to two electronic transitions. Five bands observed in the 6500-7000 cm⁻¹ region consist of R, P, and Q branches with no combination defect or Λ-doubling. They have been assigned as five sub-bands of a ΔΛ=±1 transition with Λ>1. Nine bands observed in the 9800-11 000 cm⁻¹ regions consist of R and P branches, and they are also free from Λ-doubling. These bands have been classified into four sub-bands of a ΔΛ=0 transition (with Λ>1), which has tentatively been assigned as . The two transitions have no electronic states in common. Ab initio calculations have been performed on NbCl and the spectroscopic properties of the low-lying electronic states have been calculated. The ground state of NbCl has been predicted to be a state arising from the 3σ¹ 1δ² 2π¹ configuration, with a low-lying state at 1300 cm⁻¹ from the 3σ¹ 1δ¹ 2π² configuration. The results of our experimental and theoretical studies will be presented. This work represents the first experimental investigation of the spectra of NbCl and the first ab initio prediction of the spectroscopic properties of the low-lying electronic states.     

Line parameters of HDO from high-resolution Fourier transform spectroscopy in the 11 500-23 000 cm spectral region

This work presents new measurements of HDO line parameters in the near-infrared and visible regions (11 500-23 000 cm⁻¹). The measurements consist in high-resolution Fourier transform absorption spectra of H₂O/HDO/D₂O vapor mixtures, obtained using a long absorption path. Spectra with and without nitrogen as the buffer gas were recorded. Due to the simultaneous presence of the three isotopologues H₂O, D₂O, and HDO, the H₂O lines removal and the D₂O lines identification were two necessary preliminary steps to derive the HDO line parameters. The D₂O contribution was small and confined to the well-known 4ν₁ + ν₃ band. An extensive listing of HDO spectroscopic parameters was obtained, for the first time, by fitting some 3256 observed lines to Voigt line profiles. The list contains calibrated line positions, absorption cross-sections and, for many of the lines, N₂-broadening coefficients, as well as N₂-induced frequency shifts. As a result of the low HDO vapor pressures, it was not possible to retrieve the self-broadening parameters. The list is available on the http://www.ulb.ac.be/cpm website.     

双光梳光谱学研究进展

双光梳光谱仪具有超高光谱分辨率、超快采样时间的特性,是研究分子吸收光谱的有力工具。文章介绍了双光梳光谱仪的基本原理以及它与传统傅里叶光谱仪的区别,叙述了几种典型的双光梳光谱仪构成及其用于测量分子吸收光谱的实验结果,并对双光梳光谱学做了详细说明。随着中红外光梳和中红外探测器技术的进一步发展,双光梳光谱学将会在大气污染监测、呼吸气体检测等重要的国计民生领域得到越来越广泛的应用。     

Theoretical and experimental treatment of the spectral interferences in atomic absorption spectroscopy

Spectral line interference may occur in atomic absorption spectroscopy, when there is a significant overlap of the emission line profile with the absorption line profiles of any interfering species in the flame. The presence of the spectral interference can cause errors in the measurements of the net absorbance, and in the background correction.Both theoretical calculations and experimental observations reported in this paper provide a remark of this interference effect. It can be concluded from the experimental results, that the spectral interference is located close to the resonance wavelength of the analyte.Two different examples representing various types and degrees of spectral interference are discussed.     

Fourier transform spectroscopy of new emission systems of NbN in the visible region

The emission spectrum of NbN has been reinvestigated in the 8000-35 000  cm⁻¹ region using a Fourier transform spectrometer and two groups of new bands were observed. The bands observed in the 18 000-20 000 cm⁻¹ region have been assigned to a new ³Π-X³Δ transition. Three bands with R heads near 19 463.8, 19 659.0 and 19 757.0 cm⁻¹ have been assigned as 0-0 bands of the ³Π₂-X³Δ₃, ³Π₁-X³Δ₂ and ³Π_0±-X³Δ₁ subbands, respectively, of this new transition. Three additional ΔΩ = 0 bands have been observed in the 24 000-26 000  cm⁻¹ region. A 0-0 band with an R head near 25 409.9 cm⁻¹ has been assigned as a ΔΩ = 0 transition having X³Δ₂ as its lower state while two additional bands with heads near 25 518.7 and 25 534.8 cm⁻¹ were found to be ΔΩ = 0 bands having X³Δ₁ as the common lower state. Two of these three bands are perhaps subbands of a ³Δ-X³Δ transition. Most of the excited levels are affected by perturbations.     

Effects of apodization smoothing and denoising on spectral fitting

PurposeVisual review of individual spectra in magnetic resonance spectroscopic imaging (MRSI) data benefits from the application of spectral smoothing; however, if this processing step is applied prior to spectral analysis this can impact the accuracy of the quantitation. This study aims to analyze the effect of spectral denoising and apodization smoothing on the quantitation of whole-brain MRSI data obtained at short TE.MethodsShort-TE MRSI data obtained at 3 T were analyzed with no spectral smoothing, following (i) Gaussian apodization with values of 1, 2, 4, 6, and 8 Hz, and (ii) denoising using principal component analysis (dnPCA) with 3 different values for the number of retained principal components. The mean lobar white matter estimates for four metabolites, signal-to-noise ratio (SNR), spectral linewidth, and confidence intervals were compared to data reconstructed using no smoothing. Additionally, a voxel-wise comparison for N-acetylaspartate quantitation with different smoothing schemes was performed.ResultsSignificant pairwise differences were seen for all Gaussian smoothing methods as compared to no smoothing (p<0.001) in linewidth and metabolite estimates, whereas dnPCA methods showing no statistically significant differences in these measures. Confidence intervals decreased, and SNR increased with increasing levels of apodization smoothing or dnPCA denoising.ConclusionMild Gaussian apodization (≤2 Hz at 3 T) can be applied with minimal (1%) errors in quantitation; however, smoothing values greater than that can significantly affect metabolite quantification. In contrast, mild to moderate dnPCA based denoising provides quantitative results that are consistent with the analysis of unsmoothed data and this method is recommended for spectral denoising.     

Identification and treatment approach for spectral baseline distortion in processing of gas analysis online by Fourier transform infrared spectroscopy

Addressing the problem of baseline drift or distortion in Fourier transform infrared spectra when analyzing gas continuously at scene, a novel and simple method is proposed to identify and treat spectral baseline drift. Firstly, the spectral baseline was corrected with the common approach followed by standard gas analysis. Then, absorbance of analytes was reconstructed and reduced from the corrected spectrum. Finally, a Fourier transform was performed with the difference spectrum, and the first four-line strengths were used as feature variables to determine if baseline distortion occurred. In the end of this paper, this approach has been applied in the field of wellhead gas logging with Fourier transform infrared. The application results showed that all the spikes had been removed from the raw logging curves, confirming that this approach could be used to identify spectral baseline distortion accurately.     

基于傅里叶变换红外光谱法CO2气体碳同位素比检测研究

介绍了基于傅里叶变换红外技术检测CO₂气体碳同位素比的新方法, 详细介绍了如何从HITRAN红外数据库中提取气体标准吸收截面; 介绍了基于非线性最小二乘法反演CO₂气体碳同位素比和整套实验装置的组成及实验步骤. 从理论和实验分析两方面讨论了温度和气压变化对δ¹³CO₂值的影响规律. 对于同一CO₂标准气体, 采用FTIR和同位素质谱法两种技术进行了δ¹³CO₂值对比检测, 两种测量技术的平均值差异仅为0.25%. 从实验结果可以看出, FTIR技术可以实现对CO₂气体碳同位素比的检测.     

Evaluation of spectral phase in spectrally resolved white-light interferometry: Comparative study of single-frame techniques

Spectral phase in a white-light interferogram contains information about the absolute optical path difference (OPD) in the interferometer. Evaluation of spectral phase is therefore important in applications such as profilometry with white light. In spectrally resolved white-light interferometry (SRWLI) the white-light interferogram is spectrally decomposed by a spectrometer in order to determine this phase. Several single-frame techniques in SRWLI have been proposed for the evaluation of the phase including Fourier transform, Hilbert transform, spatial phase shifting, windowed Fourier transform and wavelet transform. In this paper we present a comparative study of these techniques in this application in relation to the temporal phase-shifting technique which is a multi- frame method. Further, we also propose a modified method to remove the influence of source spectrum modulation in Hilbert transform procedure.     

Rotational spectroscopy of the first excited state of the acetylenic C-H stretch of 3-fluoropropyne performed by infrared-Fourier transform microwave-microwave triple-resonance spectroscopy

The rotational spectra of 3-fluoropropyne in the ground and first excited acetylenic C-H stretch vibrational state have been measured. The pure rotational spectrum of the normal species and the ¹³C isotopomers were measured using FTMW-cwMW double-resonance spectroscopy based on the Autler-Townes (AC Stark) effect. The lineshape properties of this measurement make it possible to determine the transition strength, ΔJ-selection rules, and the relative energy ordering of the quantum states. The frequency accuracy of this technique is tested against a previous pure rotational study of 3-fluoropropyne. The rotational spectrum of vibrationally excited state was obtained through IR-FTMW-cwMW methods. In this technique a single-longitudinal-mode pulsed infrared laser source vibrationally excites the acetylenic C-H stretch with J-selectivity. The rotational spectrum of the excited state is then obtained by FTMW and FTMW-cwMW double-resonance methods. The excited-state measurements have a signal-to-noise ratio comparable to the pure rotational spectrum. The residuals in the excited-state fit are larger than those obtained in the ground-state fit. This greater deviation from a standard asymmetric top spectrum is most likely due to weak perturbations to the acetylenic C-H spectrum.     

基于傅里叶变换光谱技术的Zoom-FFT算法研究

在分析经典的Zoom-FFT算法基础上,提出一种基于傅里叶变换光谱技术的Zoom-FFT算法,用matlab仿真常规FFT算法和Zoom-FFT算法,对不同采样步长的干涉条纹进行数据处理,通过反演出的光谱曲线图和原始光谱曲线图可以看出:采样步长小于20 m时,FFT和Zoom-FFT算法都可以反演出光谱;而当采样步长大于20 m且小于33.3 m时,FFT算法未能反演出光谱,而Zoom-FFT算法仍然可以反演出光谱。     

13C and 15N spectral editing inside histidine imidazole ring through solid-state NMR spectroscopy

Histidine usually exists in three different forms (including biprotonated species, neutral τ and π tautomers) at physiological pH in biological systems. The different protonation and tautomerization states of histidine can be characteristically determined by ¹³C and ¹⁵N chemical shifts of imidazole ring. In this work, solid-state NMR techniques were developed for spectral editing of ¹³C and ¹⁵N sites in histidine imidazole ring, which provides a benchmark to distinguish the existing forms of histidine. The selections of ¹³C_γ, ¹³C_δ2, ¹⁵N_δ1, and ¹⁵N_ε2 sites were successfully achieved based on one-bond homo- and hetero-nuclear dipole interactions. Moreover, it was demonstrated that ¹H, ¹³C, and ¹⁵ chemical shifts were roughly linearly correlated with the corresponding atomic charge in histidine imidazole ring by theoretical calculations. Accordingly, the ¹H, ¹³C and ¹⁵N chemical shifts variation in different protonation and tautomerization states could be ascribed to the atomic charge change due to proton transfer in biological process.     

Impact of scene changes during data acquisition in Fourier spectroscopy

The impact of changes of the spectral radiance in a Fourier transform spectrometer's field of view (FOV) during recording time of an interferogram is examined. Particularly systematic changes, with a time constant which is in the order of the time required to record one interferogram, are of interest here. Both an analytic and a simulative approach is made to examine the impact on the measured spectrum. Airborne measurements of fire scenes are a typical example where changes of spectral radiance during data acquisition time are inevitable. In such cases, flight velocity causes the scene in the spectrometer's FOV to change continuously, and the scene elements (fire, for example) might be expected to be dynamical even by themselves.