Speed-dependent Voigt Profile Research Articles

Line-shape parameters and their temperature dependence for self-broadened CO2 lines in the 296 K- 1250 K range by requantized classical molecular dynamics simulations

Line-shape parameters for self-broadened CO2 transitions are predicted for temperatures ranging from 296 K to 1250 K, using requantized molecular dynamics simulations (rCMDS). The line broadening coefficient, the speed dependence component and the first-order line-mixing coefficient for lines with rotational quantum number from 2 to 100, have been determined from fits of the rCMDS spectra with the Voigt and speed dependent Voigt profiles. These parameters and their temperature dependence were compared with recent high-quality measurements at both room and high temperatures, showing good agreements for all considered parameters. In particular, this study highlights that the temperature dependence of the speed dependent Voigt line broadening coefficient in the HITRAN database needs to be corrected. Additionally, we demonstrate that the temperature dependence for the speed-dependence of the line broadening differs from that of the line broadening, contrary to the assumption widely used in the literature. These findings confirm the quality of theoretical predictions using rCMDS. The data provided can be used to complete and improve spectroscopic databases for various applications.

Line intensities of CO near 1560 nm measured with absorption and dispersion spectroscopy

Abstract High-precision line intensities are of great value in various applications, such as greenhouse gas metrology, planetary atmospheric analysis, and trace gas detection. 
Here we report simultaneous measurements of cavity-enhanced absorption and dispersion spectroscopy of the prototype molecule $^{12}$C$^{16}$O using the same optical resonant cavity. Nine lines were measured in the R branch of the $v=3-0$ band. The absorption and dispersion spectra were fitted separately with speed-dependent Voigt profiles, and the line intensities obtained by the two methods agree within the experimental uncertainty of about 1\textperthousand. 
The results demonstrate the feasibility of SI-traceable molecular density measurements based on laser spectroscopy.

Water-vapor absorption database using Dual Comb Spectroscopy from 300 to 1300 K Part II: Air-Broadened H2O, 6600 to 7650 cm−1

We present broadband dual frequency comb laser absorption measurements of 2 % H2O (natural isotopic abundance of 99.7 % H216O) in air from 6600 to 7650 cm−1 (1307–1515 nm) with a spectral point spacing of 0.0068 cm−1. Twenty-nine datasets were collected at temperatures between 300 and 1300 K (±0.82 % average uncertainty) and pressures ranging from 20 to 600 Torr (±0.25 %) with an average residual absorbance noise of 8.0E-4 across the spectrum for all measurements. We fit measurements using a quadratic speed-dependent Voigt profile to determine 7088 absorption parameters for 3366 individual transitions found in HITRAN2020. These measurements build on the line strength, line center, self-broadening, and self-shift parameters determined in the Part I companion of this work. Here we measure air-broadened width (with temperature- and speed-dependence) and air pressure shift (with temperature-dependence) parameters. Various trends are explored for extrapolation to weak transitions that were not covered in this work. Improvements made in this work are predominantly due to the inclusion of air pressure shift temperature dependence values. In aggregate, these updates improved RMS absorbance error by a factor of 4.2 on average, and the remaining residual is predominantly spectral noise. This updated database improves high-temperature spectroscopic knowledge across the 6600-7650 cm−1 region of H2O absorption.

Refined line-shape parameters for CO lines broadened by air predicted from requantized classical molecular dynamics simulations

Line-shape parameters and their temperature dependences for CO lines broadened by air have been predicted using requantized classical molecular dynamics simulations. The latter were carried out for mixtures of CO in air for five temperatures ranging from 200 K to 400 K, all at a pressure of 1 atm. For each temperature and pressure condition, various Doppler widths have been considered enabling the calculations of absorption spectra for a large range of collisional to Doppler widths ratios. The simulated spectra were then fitted with the speed-dependent Nelkin-Ghatak, speed-dependent Voigt and Voigt profiles, providing the corresponding line-shape parameters. First-order line-mixing parameters were also retrieved. The temperature dependences of these parameters were then determined using a single power law. Comparisons between the predicted parameters and available experimental values as well as those of the latest edition of the HITRAN database show good agreements. The theoretically predicted values and their temperature dependences can thus be used for the modeling of the absorption spectrum of CO in air and to complete spectroscopic databases when accurate experimentally-determined values are not available.

Water-vapor absorption database using dual comb spectroscopy from 300 to 1300 K part I: Pure H2O, 6600 to 7650 cm-1

We present broadband dual frequency comb laser measurements of pure H2O absorption (natural isotopic abundance of 99.7 % H216O) from 6600 to 7650 cm−1 (1307–1515 nm) with a spectral point spacing of 0.0068 cm−1. Twenty-nine absorption spectra were collected at temperatures between 300 and 1300 K (±0.81 % average uncertainty) and pressures ranging from 0.5 to 16 Torr (±0.27 %) with an average residual absorbance noise of 7.4E-4 across the spectrum for all measurements. We fit measurements using a quadratic speed-dependent Voigt profile to determine 17,030 absorption parameters for 5986 transitions found in HITRAN2020. The measured parameters include line strength, line center, self-broadening, and self-shift, along with their power-law temperature-dependence exponents, and speed-dependent self-width parameters. A second version of the retrieved parameters with fits that do not include speed-dependence is also presented. We explore various trends to enable parameter extrapolation for weak transitions that were not covered in this work. We measured line parameters for an additional 614 features not presently catalogued in the HITRAN2020 database. In aggregate, these updates improved RMS absorbance error by a factor of 2.6 on average, and the remaining residual is predominantly spectral noise. This updated database improves high-temperature spectroscopic knowledge across the 6600–7650 cm−1 region of H2O absorption. An accompanying paper (Part II) describes similar measurements and a new database for air-broadened H2O absorption.

CRDS line-shape study of the (7–0) band of CO

We present the results of the spectral line-shape study of the first measurement of the extremely weak (7–0) band of the 12C16O molecule. Measurements were done with a highly sensitive cavity ring-down spectrometer. Collisional narrowing, analyzed in terms of speed-dependent effects, was observed for the first time for transitions with line intensities below 2⋅10−29 cm/molecule at 296 K. We provide a full set of line-shape parameters of the speed-dependent and regular Voigt profile analysis for 14 transitions from P and R branches. Experimental verification of a strong vibrational dependence of the pressure shifting described by the Hartmann model (Hartmann, 2009) is extended up to the sixth overtone highly sensitive to the model parameter.

High-temperature line strength and line shape parameters measurements of Ar- and N2-perturbed CO2 lines near 4.18 µm in a shock tube

Line strengths and line shape parameters of Ar- and N2-perturbed CO2 R-branch transitions (82 ≤J″≤ 90) in the ν3 fundamental band were measured in a shock tube from 730 K to 2500 K and pressure below 1.13 atm using laser absorption spectroscopy (LAS). The retrieved absorption curves were fitted with the Voigt and the quadratic speed-dependent Voigt (qSDV) profiles to obtain the line strengths, broadening, and shift coefficients. Line strengths were compared to values in HITEMP, HITRAN2020, and Ames2021 databases, and the HITEMP shows the best agreement with the measured results. Ar- and N2-broadening and shift coefficients were regressed into the power law and the double power law form to study the temperature dependence of the targeted lines. The speed-dependence parameter aw was temperature-dependent. The J-dependence of the derived broadening and shift coefficients was discussed. The results of this work will aid the development of the absorption model in the far-wing region and the design of CO2 LAS sensors for high-temperature measurement.

CO2-broadened lineshapes in the oxygen A-band

High-resolution spectra of O2 broadened by CO2 pressure with high signal/noise ratio (2500 at the absorbance equal 1) were recorded with a Bruker IFS 125 FTIR spectrometer in the 13,000–13,260 cm−1 spectral region. The multispectrum fitting procedure has been applied to recover the spectral line parameters from measurements using the Voigt and speed-dependent Voigt (SDV) profiles. The O2 absorption line parameters for 40 transitions were measured. The parameters for SDV profile were studied for the first time. The uncertainty of the broadening coefficients for the unblended O2 absorption lines was estimated to be <3 %. The determined parameters were compared to HITRAN2020 and literature data. The values of the self-broadening coefficients and intensities were in good agreement with literature data.

SI-traceable validation of a laser spectrometer for balloon-borne measurements of water vapor in the upper atmosphere

Abstract. Despite its crucial role in the Earth's radiative balance, upper-air water vapor (H2O) is still lacking accurate, in situ, and continuous monitoring. Especially in the upper troposphere–lower stratosphere (UTLS), these measurements are notoriously difficult, and significant discrepancies have been reported in the past between different measuring techniques. Here, we present a laboratory assessment of a recently developed mid-IR quantum-cascade laser absorption spectrometer, called ALBATROSS, for balloon-borne measurements of H2O in the UTLS. The validation was performed using SI-traceable reference gas mixtures generated based on the permeation method and dynamic dilution. The accuracy and precision of ALBATROSS were evaluated at a wide range of pressures (30–250 mbar) and H2O amount fractions (2.5–35 ppm), representative of the atmospheric variability in H2O in the UTLS. The best agreement was achieved by implementing a quadratic speed-dependent Voigt profile (qSDVP) line shape model in the spectroscopic retrieval algorithm. The molecular parameters required by this parameterization were determined empirically using a multi-spectrum fitting approach over different pressure conditions. In the laboratory environment, ALBATROSS achieves an accuracy better than ±1.5 % with respect to the SI-traceable reference at all investigated pressures and H2O amount fractions. The measurement precision was found to be better than 30 ppb (i.e., 0.1 % at 35 ppm H2O) at 1 s resolution for all conditions. This performance, unprecedented for a balloon-borne hygrometer, demonstrates the exceptional potential of mid-IR laser absorption spectroscopy as a new reference method for in situ measurements of H2O in the UTLS.

Accurate determination of line intensity of H[formula omitted]O near 7181 cm−1 : SI-traceable measurement using primary trace-moisture standard in N[formula omitted] gas

The absorption spectra of trace water vapor in N2 gas with a mole fraction of 500 nmol/mol in the pressure range of 20 kPa to 140 kPa were obtained near 7181 cm−1 using wavelength-meter-controlled cavity ring-down spectroscopy (WMC-CRDS). The reference gas (water vapor in N2 gas) was produced using a generation system of primary measurement standards for trace moisture in gases, where the amount of water vapor was measured using a gravimetric method. The obtained spectra of H2O were analyzed using a multispectrum fitting technique with a speed-dependent asymmetric Voigt profile (SDAVP). The values of the line intensity were determined with traceability to the International System of Units (SI). We could achieve the expanded uncertainty below 2 % for the line intensities of the strong lines.

Measurements and calculations of CO2-broadening and shift coefficients of water vapor transitions in the 5150–5550 cm−1 spectral region

The high-resolution H2O absorption spectra induced by CO2 pressure have been recorded using a Bruker IFS 125 HR FTIR spectrometer at room temperature in the 5150–5550 cm–1 spectral region. A multispectrum fitting procedure has been applied to these spectra to recover the spectral line parameters using the speed-dependent Voigt profile. The water vapor line broadening and shift coefficients for lines of two vibrational bands ν2+ν3 and ν1+ν2 were obtained. An uncertainty of the broadening coefficients for the unblended H2O absorption lines was estimated to be 2–3%. The calculations of line-broadening and line-shifting parameters were performed using the semi-empirical method which is based on the impact theory and enables a realistic prediction modified by introducing additional parameters obtained by using empirical data. The root mean square deviations of measured and calculated line widths and shifts were 0.0086 and 0.0053 cm−1 atm−1, respectively.

Measurements of air-broadening parameters of water vapour transitions in the 5090–7490 cm−1 spectral region

High-resolution spectra of H2O with a high signal/noise ratio (2500 at the absorbance equal 1) were recorded with a Bruker IFS 125 FTIR spectrometer covering H2O transitions from 5090 to 7490 cm−1 at the spectral resolution of 0.01 cm−1. The multispectrum fitting procedure has been applied to recover the spectral line parameters from measurements using quadratic speed-dependent Voigt (qSDV) profiles which made it possible to describe the experimental profile at the noise level. The H2 16O line broadening and shift coefficients for 439 lines of 7 vibrational bands 2ν3, ν2 + ν3, 2ν2 + ν3, ν1 + ν3, ν1 + ν2, ν1 + 2ν2 and 2ν1 were measured. A total of 339 lines were studied for the first time. The uncertainty of the broadening coefficients for the unblended H2O absorption lines was 2–4%. The determined parameters are compared to HITRAN12 and experimental values. The ratio between HITRAN data and our broadening coefficient varied from 0.82 to 1.21. The calculations of broadening and shift coefficients were performed in a framework of the semi-classical method and by using an empirical function whose parameters were taken from the literature. The uncertainty of calculations was 5% for the air broadening half-widths on average.

Temperature behavior of collisional parameters of oxygen fine-structure lines: O2-O2 case

The microwave spectrum of oxygen, which consists of the fine-structure lines, is a tool for atmospheric research. An accurate line shape profile is necessary for modeling of the atmospheric absorption and interpreting the radiometric data. In this study, more than 500 recordings of the oxygen millimeter wave lines were obtained in a wide range of pressures (0.3–1500 Torr) and temperatures (235–357 K) using two experimental techniques that differ in principle of operation. High quality of the experimental spectra and negligible baseline allowed accurate analysis of the line shape using the quadratic speed-dependent Voigt profile. Line shape parameters for six fine-structure oxygen lines (N = 1+,1−, 7+, 11+, 15+, and 19−), including self-broadening and self-shifting, were refined. The parameters of the speed dependence of the collision relaxation rate demonstrated significantly different temperature behavior for different rotational states. In particular, the ratio γ2/γ0 reveals temperature independence for the lowest rotational state; the dependence becomes notable with the growth of rotational excitation. For practical use, the empirical functions were fitted to the experimental temperature and rotational dependences of the line shape parameters.

Line shape parameters of air-broadened 12CO2 transitions in the 2.0 µm region, with their temperature dependence

The 20013–00001 and 20012–00001 bands of 12C16O2 in the 2 µm region are used by many instruments on-board satellites and in ground-based networks to monitor the CO2 column-averaged dry air mole fraction in the Earth's atmosphere. An accurate knowledge of the corresponding spectroscopic parameters and their temperature dependence is thus needed to map sources and sinks of carbon dioxide. In this work, we retrieve the line-shape parameters and their temperature dependence exponents/coefficients for the P(16) and P(28) transitions of the 20012–00001 band, and the R(24) and R(30) transitions of the 20013–00001 band. These parameters are obtained from a multi-spectrum fit procedure using a speed-dependent Nelkin-Ghatak profile including the line-mixing effect in its first-order approximation. High signal-to-noise spectra (quality factor between typically 3000 and 6000) are recorded at different conditions of pressure (from 50 to 750 Torr) and temperature (from 245 to 330 K) for mixtures of CO2 in air using a comb-assisted cavity ring down spectrometer with a temperature stabilized high finesse cavity. Air-broadening coefficients are measured with an estimated uncertainty better than 0.1% and absolute frequencies of the transitions are obtained with uncertainty better than 100 kHz. The determined line-shape parameters are consistent with previous values derived in the 1.6 µm region confirming the absence of significant vibrational dependence (except for the air-pressure shift coefficient). The retrieved parameters agree well with literature values by Fourier transform spectroscopy and classical molecular dynamic simulations. The parameters of the speed-dependent Voigt profiles provided by the HITRAN2020 database are mostly validated. The comparison to the CO2 line parameters adopted for the OCO missions is discussed.

Line-shape parameters and line mixing for the low-intensity high-J oxygen transitions

We present the results of investigations of the oxygen B-band R-branch transitions with intensities in the range from 1.5×10−26 to 2.5×10−28 cm−1/(molecule/cm2) and the total angular momentum quantum number for the lower state, J”, up to 38. High resolution CRDS laboratory spectra were fitted with the Voigt profile as well as with advanced line-shape functions consistent with the Hartmann–Tran profile, that is the speed-dependent Voigt profile and speed-dependent Nelkin–Ghatak profile using the multispectrum fitting technique. The line positions, line intensities and other line-shape parameters, together with our previous data, were collated to show trends for the whole R branch. Using the determined frequencies of the B-band transitions, the Dunham fit was performed to obtain spectroscopic parameters for both ground and excited states. Newly recorded spectra at the band head, with the signal-to-noise ratio above 20000, were analyzed with line profiles including also the line mixing. First-order line mixing determined directly from the line shape fitting at relatively low pressure (32 Torr) is in good agreement with values calculated by Sung et al. [1].

High precision measurement of spectroscopic parameters of H&lt;sub&gt;2&lt;/sub&gt;S in 6320—6350 cm&lt;sup&gt;–1&lt;/sup&gt; band

As a highly corrosive and highly toxic gas, hydrogen sulfide (H&lt;sub&gt;2&lt;/sub&gt;S) is an important intermediate product or pollutant in many fields such as chemical industry, energy and environment. Accurate online measurement of its concentration is of great significance for process control and production safety. Tunable diode laser absorption spectroscopy (TDLAS), as a quantitative absorption spectroscopy technique, is suitable for high-precision on-line measurement of H&lt;sub&gt;2&lt;/sub&gt;S concentration in atmospheric environmental monitoring and industrial processes control. Considering that most of the spectroscopic parameters of H&lt;sub&gt;2&lt;/sub&gt;S in the HITRAN2020 database are mainly calculated based on semi-empirical theoretical model and the experimental data to support them are lacking. In this work, direct absorption spectroscopy (DAS) method is firstly used to measure the absorption spectra of H&lt;sub&gt;2&lt;/sub&gt;S in the band of 6320–6350 cm&lt;sup&gt;–1&lt;/sup&gt;. Six groups of characteristic lines with strong absorption and relative independence are selected as the target transitions for experimental measurement. Then, the wavelength modulation-direct absorption (WM-DAS) method with no calibration and high signal-to-noise ratio is used to measure the absorbances of the six groups of transitions under different pressures. Voigt, Raution and quadratic speed-dependent Voigt profiles fit the measured absorbances by least squares method in order to obtain the spectroscopic parameters such as the collision broadening coefficient, line strength and Dicke narrowing coefficient. And the minimum standard deviation of residual error of absorbances is 7×10&lt;sup&gt;–5&lt;/sup&gt;. The measurement uncertainty of each line strength is less than 2%, and the uncertainty of collision broadening coefficients, Dicke narrowing coefficients and the speed-dependent coefficients are all less than 10%. This work is helpful in improving the H&lt;sub&gt;2&lt;/sub&gt;S spectral database and providing the spectral data basis for the high-precision measurement of H&lt;sub&gt;2&lt;/sub&gt;S concentration.

Water vapor absorption line parameters in the 6760–7430 cm–1 region for application to CO2-rich planetary atmosphere

The absorption spectra of the mixtures of water vapor and carbon dioxide at five different partial pressures of both gasses have been recorded using the Bruker IFS 125 HR FTIR spectrometer at room temperature in the 6760–7430 cm–1 spectral region. The multispectrum fitting procedure has been applied to these spectra to recover the spectral line parameters. To obtain the spectral lines parameters the quadratic speed-dependent Voigt profile was used. The water vapor line broadening and shift coefficients for lines of three vibrational bands ν1+ν3, 2ν2+ν3, and 2ν1 were obtained.

CO&lt;sub&gt;2&lt;/sub&gt;-broadened coefficients of water vapor molecule in 1.1 μm band

The absorption spectral parameters of water vapor molecules are the key basic scientific data for the remote sensing detection and the planetary observation applications. Based on a narrow line-width external cavity diode laser and a long-path absorption cell, 18 absorption spectral lines of CO&lt;sub&gt;2&lt;/sub&gt;-broadened water vapor molecules in a 9332–9722 cm&lt;sup&gt;–1&lt;/sup&gt; range at room temperature are measured. To obtain the CO&lt;sub&gt;2&lt;/sub&gt;-broadened water vapor molecule coefficients, the Voigt profile and the quadratic speed-dependent Voigt profile are used to fit the absorption spectrum data. The quadratic speed-dependent Voigt profile shows better fitting capability. Comparing with the air-broadened coefficients of the corresponding region from the HITRAN2020 database, the mean ratios of the CO&lt;sub&gt;2&lt;/sub&gt;-broadened coefficients of water vapor molecules and the air-broadened coefficients obtained from the two models of the line shape are 1.327 and 1.454, respectively, which verifies that the method of estimating the CO&lt;sub&gt;2&lt;/sub&gt;-broadened coefficient by the air-broadened coefficient of water vapor molecules has certain reliability. This study can provide reference data of measured spectral parameters for the detection technology and related research of atmospheric structures of Mars and Venus in the near-infrared region.

Spectral analysis of H2O near 7180 cm–1 to accurately measure trace moisture in N2 gas: evaluation of line shape profiles using Akaike Information Criterion

The absorption spectra of H2O in N2 gas were measured at atmospheric pressure and room temperature near 7180 cm–1 in the moisture range of 9.7–148.7 nmol mol−1 in mole fraction using cavity ring-down spectroscopy. The line shape profiles used for the spectral analysis for the trace moisture measurement were evaluated based on the Akaike Information Criterion as well as the fitting residuals. The optimal line shape profile in this study was the speed-dependent asymmetric Voigt profile (SDAVP). From the comparison of the fitting results calculated using the Lorentzian profile (LP) with those calculated using the SDAVP, the relative difference in the integrated line area between the two line shape profiles was found to be systematic and constant. The measurement error due to the use of the LP could be corrected using a correction factor of 1.00479 ± 0.00077.

High-temperature absorption line shape parameters for CO2 in the 6800–7000 cm-1 region from dual frequency comb measurements up to 1000 K

We present the first systematic study of self-broadened CO2 line shape parameters in the 00031 ← 00001 and 01131 ← 01101 bands at high temperature. We report temperature-dependent broadening coefficients and pressure shift coefficients for more than 40 transitions in the 00031 ← 00001 band, and 75 transitions in the 01131 ← 01101 hot band. The majority of these parameters have never been measured at high temperature. The new parameters are determined from 12 spectra measured with a broadband, high-resolution dual frequency comb absorption spectrometer in a high-temperature gas cell from room temperature to 980 K. We fit the measured spectra using a multispectrum fitting approach to derive line shape parameters with both a Voigt and speed-dependent Voigt profile. We compare the measured parameters to calculated line shape parameters as well as to the new parameter set developed for the HITRAN2020 database. The new measurements demonstrate strong agreement with HITRAN2020 (within 1% in some cases), and provide new measurements of the temperature dependence of the broadening and pressure shift coefficient that will improve the accuracy of the database at high temperature. We extend our results to high |m| transitions using a set of empirical models for the broadening and pressure shift coefficients, which we fit to our data and validate for high-temperature transitions up to |m| = 98. We use these models to construct a set of database files that can be used to supplement the HITRAN database for self-broadened CO2 in the 6800 – 7000 cm-1 region. These new parameters significantly improve the accuracy of absorption models for high-temperature CO2 in this frequency range, and also provide useful data to test and improve methods used to calculate CO2 line shape parameters.