Line Shape Model Research Articles

Searching for a second excitation in the inelastic neutron scattering spectrum of a liquid metal: a Bayesian analysis

When probed at nanometer and picosecond scales, the properties of a liquid present striking analogies with the ones of the corresponding solid, one of the most surprising is the ability of supporting shear wave propagation, as a rigid medium. Although this evidence is being reported by a growing number of terahertz scattering measurements, it remains an open question whether it is universal or rather typical of some liquids only. Furthermore, given its elusive signatures in the scattering signal, the detection of this effect appears as a typical case where an unintentional “bias of confirmation” can mislead experimentalists. We thus decided to use a Bayesian inference approach to achieve a probabilistically grounded and evidence-based lineshape modeling of the inelastic neutron scattering spectra from liquid silver, whose simulated density autocorrelations bear evidence of a shear mode propagation over very short distances. The result of our analysis indicates that the observation of any additional, non-longitudinal, acoustic modes in this simple system goes beyond the accuracy of the used scattering method.

High-spectral-resolution lidar for measuring tropospheric temperature profiles by means of Rayleigh-Brillouin scattering.

A novel high-spectral-resolution lidar receiver based on a Fizeau interferometer and a photomultiplier tube array for tropospheric temperature profiling is introduced. Compared to other temperature lidars, an imaging approach is used to resolve the entire Rayleigh-Brillouin (RB) spectrum without applying frequency scanning techniques. The functionality of the system is demonstrated by means of a nighttime measurement. Atmospheric temperature is retrieved from 4.0 km to 9.2 km by analyzing the measured RB spectra with the Tenti S6 line shape model. The systematic error of the retrieved temperatures is determined to be smaller than 3 K, and the corresponding random error varies between 1.7 K (4.0 km) and 2.3 K (9.2 km) for an observation time of 5 min and a vertical resolution of 0.3 km. Considering the short averaging time and the stable arrangement of the system, the suggested approach is also attractive for future airborne applications.

Experimental and simulated study of line-shape models for measuring spectroscopic parameters using the WM-DAS method — Part II: Broadening and narrowing of select near-infrared [formula omitted] and CO lines perturbed by Ar, [formula omitted] and He

Wavelength modulation-direct absorption spectroscopy (WM-DAS) has been used to measure the absorption line shapes of the H2O transition at 7299.431 cm−1 and the CO transition at 4297.705 cm−1 perturbed by Ar, N2 and He. The foreign-gas broadening, narrowing and pressure-shift parameters were determined from various lineshape models at pressures from 1 to 100 kPa. The measured N2-broadening coefficients are the largest among the three perturbers and the measured N2- and Ar-pressure-shift coefficients are both negative while that for He is positive. The nonlinear pressure dependence of the velocity-changing rates for the Galatry and Rautian profiles and the speed-dependent coefficients for Speed-dependent Voigt profile were all observed for the 6 binary systems. The measured velocity-changing rates were all larger than the corresponding dynamic friction coefficients in H2O-Ar/N2/He while smaller than those in CO-Ar/N2/He. The narrowing parameters were also retrieved and compared by taking both the Dicke narrowing and speed dependent effects into account. The partially Correlated quadratic Speed-dependent Hard-Collision profile was simulated and fit by the other simplified profiles to further validate the nonlinear behaviour of the narrowing parameters, which is consistent with the measured results.

Spectral line-shape analysis of CO2 transition using Hartmann-Tran profile and its asymptotic limits

In this paper, the Hartmann-Tran profile (HTP) and its asymptotic limits are employed to analyze the experimentally-measured absorption spectra of CO2 gas. The fitted results by multi-spectrum fitting are compared to those from the ordinary line-shape models. The equivalence and computational efficiency of the asymptotic limits of HTP to their ordinary counterparts are studied. The research shows that the parameters of line position ω0, intensity S, pressure broadening coefficient Γ0/p and shifting coefficient Δ0/p by HTP agree well with those from the speed-dependent hard-collision line-shapes. In comparison, the poorly-determined partial-correlation parameter η makes the fitted line-narrowing parameters less reliable.

Data Processing Optimization in Untargeted Metabolomics of Urine Using Voigt Lineshape Model Non-Linear Regression Analysis.

Nuclear magnetic resonance (NMR) spectroscopy is well-established to address questions in large-scale untargeted metabolomics. Although several approaches in data processing and analysis are available, significant issues remain. NMR spectroscopy of urine generates information-rich but complex spectra in which signals often overlap. Furthermore, slight changes in pH and salt concentrations cause peak shifting, which introduces, in combination with baseline irregularities, un-informative noise in statistical analysis. Within this work, a straight-forward data processing tool addresses these problems by applying a non-linear curve fitting model based on Voigt function line shape and integration of the underlying peak areas. This method allows a rapid untargeted analysis of urine metabolomics datasets without relying on time-consuming 2D-spectra based deconvolution or information from spectral libraries. The approach is validated with spiking experiments and tested on a human urine 1H dataset compared to conventionally used methods and aims to facilitate metabolomics data analysis.

Observation of asymmetric line shapes in precision microwave spectroscopy of the positronium 2S13→2PJ3 ( J=1,2 ) fine-structure intervals

We report new measurements of the positronium (Ps) $2^{3}S_{1}\ensuremath{\rightarrow}2^{3}P_{J}$ fine-structure intervals, ${\ensuremath{\nu}}_{J}$ ($J=0,1,2$). In the experiments, Ps atoms, optically excited to the radiatively metastable $2^{3}S_{1}$ level, flew through microwave radiation fields tuned to drive transitions to the short-lived $2^{3}P_{J}$ levels, which were detected via the time spectrum of subsequent ground-state Ps annihilation radiation. Both the ${\ensuremath{\nu}}_{1}$ and ${\ensuremath{\nu}}_{2}$ line shapes were found to be asymmetric, which, in the absence of a complete line-shape model, prevents accurate determination of these fine-structure intervals. Conversely, the ${\ensuremath{\nu}}_{0}$ line shape did not exhibit any significant asymmetry; the observed interval, however, was found to disagree with QED theory by 4.2 standard deviations.

Air-broadened N2O line-shape parameters and their temperature dependences by requantized classical molecular dynamics simulations

In this paper, we used requantized Classical Molecular Dynamics Simulations (rCMDS), corrected by the room temperature air-broadening coefficients measured in the ν3 band of N2O, to predict line-shape parameters including the broadening coefficient, its speed dependence component, the Dicke narrowing and the line-mixing coefficients, associated with various line-shape models for air-broadened N2O absorption lines. For that, we first computed the auto-correlation functions of the N2O dipole moment, responsible for the absorption transitions. The Fourier-Laplace transform of these functions directly yields the absorption spectra. Calculations were made for three temperatures 200, 250 and 296 K and 1 atm of N2O/air mixtures and for a large range of values of the ratio between the Doppler and Lorentzian widths. The obtained spectra were then fitted with the quadratic speed-dependent Voigt profile including the first-order line mixing using a multi-spectrum fitting procedure. Comparisons with values determined from room temperature high-precision measurements in the ν3 band of N2O by using the same profile [Loos et al, J. Quant. Spectrosc. Rad. Transf., 151, 2015, 300-309] show that our retrieved line broadening coefficients are overestimated by about 4%. The difference between the measured and predicted line broadening coefficients was used to empirically correct the rCMDS auto-correlation functions and thus the corresponding absorption spectra. These latter were then fitted with the Voigt, the speed-dependent Voigt, and the speed-dependent Nelkin-Ghatak profiles, all of them being associated with the first-order line-mixing approximation, providing the corresponding line-shape parameters for lines up to J′′ ≤ 59. The temperature dependences of various line-shape parameters were also deduced. The results show a very good agreement with available experimental data for all the considered parameters.

Correlation between Optical Localization-State and Electrical Deep-Level State in In0.52Al0.48As/In0.53Ga0.47As Quantum Well Structure.

The peculiar correlationship between the optical localization-state and the electrical deep-level defect-state was observed in the In0.52Al0.48As/In0.53Ga0.47As quantum well structure that comprises two quantum-confined electron-states and two hole-subbands. The sample clearly exhibited the Fermi edge singularity (FES) peak in its photoluminescence spectrum at 10–300 K; and the FES peak was analyzed in terms of the phenomenological line shape model with key physical parameters such as the Fermi energy, the hole localization energy, and the band-to-band transition amplitude. Through the comprehensive studies on both the theoretical calculation and the experimental evaluation of the energy band profile, we found out that the localized state, which is separated above by ~0.07 eV from the first excited hole-subband, corresponds to the deep-level state, residing at the position of ~0.75 eV far below the conduction band (i.e., near the valence band edge).

Line parameters of the P-branch of (30012) ← (00001) 12C16O2 band measured by comb-assisted, Pound-Drever-Hall locked cavity ring-down spectrometer

This article presents an accurate measurement of line parameters for the P-branch of (30012)←(00001) 12C16O2 band with rotational quantum number extending to -72. The spectrometer has a temperature stability of below 0.5 mK at 296 K and a cavity leakage of 0.068 mTorr/hour. An optical frequency comb is employed to measure the absolute transition frequencies. The measured data are processed using multi-spectrum fitting method with different line-shape models to retrieve line positions, intensities, and other line-shape parameters. The correlation between Dicke narrowing and speed-dependent broadening is studied. The measured parameters are compared to a series of spectroscopic databases and previously published results.

Structure factor lineshape model gives approximate nanoscale size of polar aggregates in the ionic liquid N-methyl-N-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide.

A Lorentzian lineshape model is developed and tested for the charge alternation peak in X-ray structure factors calculated from MD simulations for N-methyl-N-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide. Applying the model to published, experimental X-ray scattering data reproduces calculated cation-cation and anion-anion distances within 6% and implies that half of ionic aggregates are larger than 12.7 Å.

Orientational and low-frequency (0–450 cm−1) dynamics of methyl methacrylate: OHD-RIKES measurements and DFT calculations

This paper presents results of a study in which optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) and density functional theory (DFT) were used to glean information about the low-frequency (0–450 cm−1) intramolecular and intermolecular dynamics and picosecond reorientational dynamics of methyl methacrylate (MMA) as a function of temperature. Based on DFT calculations, the two intramolecular bands at 220 and 380 cm−1 in this spectral region are assigned, respectively, to the (O)CH3 torsional and C=C out-of-plane modes of the cis-conformer of MMA. In the 2–40 ps time range, the OHD-RIKES response of MMA can be well fit by a biexponential decay function with a fast component (relaxation time τ1) associated with intermolecular dynamics, and a slow component (relaxation time τ2) associated with collective reorientation. The temperature dependence of τ2 can be understood in terms of a modified Debye-Stoke-Einstein (DSE) equation with the rotation of MMA molecules being governed by slip boundary conditions, which is consistent with MMA being a non‑hydrogen-bonded liquid. τ1 and τ2 are found to be linearly correlated with each other, which is attributed to τ1 being dominated by structural relaxation and therefore to motional narrowing. The low-frequency broad band in the 0–200 cm−1 region of the Kerr spectrum of MMA was analyzed in terms of a multicomponent line-shape model, with the sum of the first three components being primarily associated with the intermolecular modes of MMA. With increasing temperature, the intermolecular band shifts to lower frequency and narrows with increasing temperature. The shift to lower frequency is attributed to the softening of the effective intermolecular potential due to decreasing density with increasing temperature and the narrowing of the intermolecular band due to motional narrowing. Finally, it is shown that the Kerr spectrum of MMA provides support for the interpretation of the main band of the low-frequency reduced-Raman spectrum of poly(methyl methacrylate) (PMMA) being due to the librational motion of the monomer units in the polymer. This explanation is physically reasonably based on fits of the multicomponent line-shape model to the spectra of PMMA and MMA. It is proposed that this librational motion is associated with the torsional motion of the methoxycarbonyl pendant group on the polymer chain.

Stochastic scattering theory for excitation-induced dephasing: Comparison to the Anderson-Kubo lineshape.

In this paper, we present a quantum stochastic model for spectroscopic lineshapes in the presence of a co-evolving and non-stationary background population of excitations. Starting from a field theory description for interacting bosonic excitons, we derive a reduced model whereby optical excitons are coupled to an incoherent background via scattering as mediated by their screened Coulomb coupling. The Heisenberg equations of motion for the optical excitons are then driven by an auxiliary stochastic population variable, which we take to be the solution of an Ornstein-Uhlenbeck process. Itô's lemma then allows us to easily construct and evaluate correlation functions and response functions. Focusing on the linear response, we compare our model to the classic Anderson-Kubo model. While similar in motivation, there are differences in the predicted lineshapes, notably in terms of asymmetry, and variation with the increasing background population.

The classical Doppler-broadened absorption line profile due to thermal effect

The classical Doppler-broadened absorption line profile, which is exactly derived by the classical Doppler effect and Maxwell speed distribution for the first time, is asymmetrically broadened with red-shift more than blue-shift, and its peak is red-shifted. Nonetheless, the half-width at half-maximum in the blue-shift wing is larger than that in the red-shift wing. The classical Doppler-broadened absorption line profile is different from the classical Doppler-broadened emission line profile. The classical Doppler-broadened absorption line profile is applied to disprove one of the two existing relativistic Doppler-broadened absorption line profiles. Also, line-shape models based on the classical Doppler-broadened absorption line profile are suggested for absorption line spectroscopy.

Experimental and simulated study of line-shape models for measuring spectroscopic parameters using the WM-DAS method — part I: Collisional broadening and absorption coefficients of H2O-Ar system

Wavelength modulation-direct absorption spectroscopy (WM-DAS) was used to investigate absorption line-shape models by measuring the H2O transition at 7299.431 cm−1 in the ν1+ν3 vibrational band perturbed by Ar. The partially Correlated quadratic Speed-dependent Hard-Collision profile (pCqSDHCP) was found to give the best fit with the smallest residuals. The Ar-induced broadening, narrowing coefficients and the absorption coefficients were measured using various profile models with comparisons to pCqSDHCP. The room-temperature pressure dependence of the measured parameters and their differences over the range from 0.3 kPa to 80 kPa (Lorentzian-to-Doppler broadening ratios from 0.06 to 3.5) were studied. The pCqSDHCP was also simulated and fit by the other simplified profiles to further explore the pressure dependence, which compares well with the experimental results. The pressure dependence of the broadening and absorption coefficients for Voigt profile (VP) were analyzed to provide the optimal pressure range when using VP to measure the spectroscopic parameters. The experimental and simulated results allow us to quantify the systematic errors in the pressure broadening and line intensity associated with simplified line shape models.

Fast approximations of spectral lineshapes to enable optimization of a filtered Rayleigh scattering experiment

Measuring the properties of scattered light is central to many laser-based gas diagnostic techniques, such as filtered Rayleigh scattering (FRS). Alongside the measurements, a model of the scattered light’s spectral lineshape is often used to extract quantitative information about the flow field like pressure, temperature, and velocity. In particular, the Tenti S6 or S7 model are frequently used to model the lineshape of Rayleigh–Brillouin (RB) scattered light. While accurate, it is well attested in the literature that these models can be computationally expensive when evaluated many times, for example, as part of iterative estimation or optimization routines. To overcome this, approximations of these spectral lineshape models can be used instead. In this paper, we develop a method called support vector spectrum approximation (SVSA). This method uses support vector regression and singular value decomposition to create efficient, accurate, and well-conditioned approximations of any existing spectral lineshape model. The SVSA framework improves upon existing approximation methods by allowing quick calculation of spectral lineshapes for arbitrary flow regimes with any number of input parameters over a wide range of values. We demonstrate the efficacy of SVSA in approximating coherent and spontaneous RB scattering spectra. In application, we use SVSA to optimize the design of a filtered Rayleigh scattering experiment of a complex shock-dominated flow. SVSA allows us to comprehensively minimize expected measurement uncertainty of number density and temperature for this experiment. It does this by enabling a high-resolution design of experiments that is otherwise intractable.

N2-collisional broadening coefficients of lines in the [formula omitted] band of chloromethane from low to room temperatures

Using a high-resolution tunable diode-laser spectrometer, we measured N2-collisional broadening coefficients in the ν2 band of chloromethane. Measurements have been realized for 15 lines at room temperature and 5 lines at different low temperatures. For each line under study, we have recorded spectra at 4 pressures of perturber gas ranging from 12 to 55 mbar. The experimental line profiles, at each pressure and each temperature, have been fitted with three different line shape models. From these fits, we obtained the collisional half-width and accurately deduced N2-broadening coefficients at up to 4 temperatures between 200 K and 296 K. Using the usual empirical power law, we determined the temperature dependence parameter of the N2-broadening coefficients.

Assessing the structural properties of graphitic and non-graphitic carbons by Raman spectroscopy

We study the transformation from molecular to crystalline of (non-)graphitic carbons synthesized from organic precursors by heat-treatment. Easy assessment of structural properties resulting from heat-treatment protocols is mandatory for industrial process monitoring. We demonstrate that Raman spectroscopy, in particular, the Raman lineshape analysis of G and D mode, offers quick assessment of the average sheet size of such carbons. We validate this method by performing Raman, WAXS and EPR measurements of series of resin and pitch-based carbons synthesized. The crystallite sizes of the WAXS analysis for the individual samples are related to corresponding positions and linewidths of G and D Raman modes and show excellent agreement between experiment and modelling from large sizes down to 4 nm. The theoretical master curves are independent of the precursor used in the synthesis, in contrast to models for the intensity ratio of D and G band versus size. We show that the latter are not universally valid and differ for each class of precursors. For sizes below 4 nm, our lineshape model fails as it is based on the bandstructure and phonon dispersions of ideal graphene. Thus, 4 nm corresponds to the fundamental transition from molecular to crystalline character for non-graphitic carbons.

Cryogenic atomic hydrogen beam apparatus with velocity characterization.

Precision spectroscopy of hydrogen often relies on effusive thermal atomic beams, and the uncertainty in the velocity distribution of these beams can introduce systematic errors and complicate lineshape models. Here, we present an apparatus capable of high signal-to-noise studies of these velocity distributions at cryogenic temperatures for both ground state (1S) and metastable (2S) hydrogen using a simple time-of-flight technique. We also investigate how the cryogenic nozzle geometry affects these results.

Non-uniform sampling in quantitative assessment of heterogeneous solid-state NMR line shapes.

Non-uniform sampling has been successfully used for solution and solid-state NMR of homogeneous samples. In the solid state, protein samples are often dominated by inhomogeneous contributions to the homogeneous line widths. In spite of different technical strategies for peak reconstruction by different methods, we validate that NUS can generally be used also for such situations where spectra are made up of complex peak shapes rather than Lorentian lines. Using the RMSD between subsampled and reconstructed data and those spectra obtained with uniform sampling for a sample comprising a wide conformational distribution, we quantitatively evaluate the identity of inhomogeneous peak patterns. The evaluation comprises Iterative Soft Thresholding (hmsIST implementation) as a method explicitly not assuming Lorentian lineshapes, as well as Sparse Multidimensional Iterative Lineshape Enhanced (SMILE) algorithm and Signal Separation Algorithm (SSA) reconstruction, which do work on the basis of Lorentian lineshape models, with different sampling densities. Even though individual peculiarities are apparent, all methods turn out principally viable to reconstruct the heterogeneously broadened peak shapes.

Analytical-function correction to the Hartmann–Tran profile for more reliable representation of the Dicke-narrowed molecular spectra

The β-corrected Hartmann–Tran profile (HTP) constitutes an approximation of the partially correlated quadratic speed-dependent billiard-ball profile (SDBBP) easily applicable in calculations. We extend the approach originally developed for self-perturbed molecules [Wcisło et al. J. Quant. Spectrosc. Radiat. Transf. 177, 75–91 (2016)] to systems with a wide range of perturber-to-absorber mass ratios, including those relevant for atmospheric studies. This approach combines the computational simplicity of the HTP with the more physically justified rigid-sphere model for velocity-changing collisions. It is important for the analysis of high-resolution spectra influenced by the Dicke-narrowing effect. The β-corrected HTP enables high quality analytical representation of experimental spectra without incurring the high computational cost of more advanced line-shape models. This correction is directly applicable to any other line-shape model based on the hard-collision model for velocity-changing collisions.