Speed-dependent Nelkin-Ghatak Profile Research Articles

Revising the H2 16O line-shape parameters around 1.1 μm based on the speed-dependent Nelkin–Ghatak profile andthe Hartmann–Tran profile

Accurate spectroscopic data for H2 16O in the 1.1 μm region are particularly important for the study of Earth’s atmosphere. The pure water vapor molecular spectra were measured based on direct laser absorption spectroscopy using a narrow line-width external cavity diode laser combined with a high-precision Fabry–Pérot etalon. A total of 31 H2 16O transitions were studied for the first time by using the speed-dependent Nelkin–Ghatak profile and the Hartmann–Tran profile. From an accurate line-shape analysis, we obtained the line intensities and the self-broadening coefficients, and they are compared with the available data reported in the HITRAN 2016 database and the HITRAN 2020 database. Finally, we obtained information on the influence of Dicke narrowing, as well as the correlations between Dicke narrowing and speed dependence, and of speed-dependent effects.

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].

Validation of spectroscopic data in the 1.27 µm spectral region by comparisons with ground-based atmospheric measurements

Various spectroscopic data for absorption lines due to the magnetic dipole transitions of the a1Δg−X3Σg−(0−0) band of O2 centered at 1.27 µm are tested by comparison with high-resolution ground-based atmospheric measurements recorded by Fourier Transform Spectrometers at Park Falls and Caltech (USA). This band is of importance for atmospheric remote sensing since it will be used (together with the O2 A-band near 760 nm) by the passive short wave infrared spectrometer onboard the MicroCarb satellite mission (i.e. this band includes the B4 band of MicroCarb, from about 7800 cm−1 to 7912 cm−1) for the determination of surface pressure and atmospheric aerosols. Spectroscopic data of the HITRAN2016 and GEISA2015 databases as well as those from recent laboratory studies are here used in a radiative transfer code to simulate atmospheric transmissions under the conditions of the measurements. Comparisons are made for different solar zenith angles and for the whole B4 spectral range considered by MicroCarb. Spectroscopic data of water vapor are also tested by considering both relatively dry and humid atmospheric conditions. Averaging the “calculated-observed” residuals over numerous recordings made for close values of the solar zenith angle and humidity enables reduction of the uncertainties due to the radiometric noise of the instrument and to the imperfect description of the atmospheric state. This enables the detection of systematic differences in the spectral residuals caused by small changes in spectroscopic data. The results show that the spectroscopic parameters in the HITRAN2016 and GEISA2015 databases lead to large residuals while data of two recent laboratory studies, obtained from spectra measured with the cavity ring-down spectroscopy technique using the speed-dependent Nelkin-Ghatak profile lead to much better agreement with atmospheric measurements. Significant residuals are noted for water vapor absorption lines simulated using parameters provided by both the HITRAN and GEISA databases.

Transition frequencies in the (2-0) band of D2 with MHz accuracy

Series of spectra of the S(0) and Q(1)-Q(4) transitions of the (2-0) band of D2 near 5860 cm−1 are recorded by comb-referenced cavity ring down spectroscopy, at four pressures up to 200 Torr. After averaging, high resolution (30 MHz step) and low noise (noise equivalent absorption, αmin≈ 1.5 × 10−11 cm−1) spectra are obtained for each pressure value. This leads to signal to noise ratios of several hundred for the considered very weak electric quadrupole transitions with intensities ranging between 4 × 10−29 and 4 × 10−28 cm/molecule. The Q(3) and Q(4) transitions are newly detected.A multi-spectrum treatment of the D2 line profiles at different pressures performed using a quadratic speed-dependent Nelkin-Ghatak profile leads to residuals close to the noise level. Considering statistical fit errors and possible biases, total uncertainties on the frequencies extrapolated at zero pressure are estimated to be between 0.30 MHz and 1.83 MHz. The derived experimental frequencies and intensities of (2-0) transitions are gathered with literature values and compared to ab initio values reported in J. Komasa, M. Puchalski, P. Czachorowski, G. Łach, K. Pachucki, Phys. Rev. A 100, 032519 (2019). An agreement within the combined uncertainties is achieved for the S(0), Q(1), Q(2) and Q(3) transitions. Line intensities are also discussed in relation with literature values.

Line-shape parameters and their temperature dependences predicted from molecular dynamics simulations for O2- and air-broadened CO2 lines

Requantized classical molecular dynamics simulations (rCMDS) were performed for CO2 highly diluted in O2 at 200, 250, 296 and 350 K using a site-site intermolecular potential. The simulations were made for 0.5 atm of O2 pressure and for a large range of Doppler widths, covering near-Doppler regime to collisional-dominant regime. The Fourier-Laplace transform of the auto-correlation functions of the dipole moment, calculated by rCMDS, leads to the associated spectra of CO2 broadened by O2. Different effects of collisions between CO2 and O2 molecules are included in the simulated spectra. In order to determine the profile parameters of O2-broadened CO2 lines, the rCMDS-calculated spectra were fitted with the speed-dependent Nelkin–Ghatak profile associated with the first-order line mixing. The collisional line broadening coefficient, its speed dependence component, the Dicke narrowing and the first-order line-mixing parameters were retrieved for lines with J up to 50 and for all considered temperatures. The temperature dependences of these line-shape parameters were then deduced using the usual single power law. From results obtained in this work and those obtained for CO2 in N2 [Nguyen et al., J Chem Phys,149, 224301, 2018], the air-broadened line-shape parameters and their temperature dependences for CO2 lines were calculated and compared with literature data showing very good agreement.

High sensitivity spectroscopy of the O2 band at 1.27 µm: (I) pure O2 line parameters above 7920 cm−1

The atmospheric band of O2 near 1.27 µm plays an important role in determining the sounded air-mass from ground or space borne atmospheric spectra. This band consists of narrow absorption lines of the a1Δg−X3Σg−(0−0) transitions superimposed to a much broader collision-induced absorption structure. The present contribution is part of a long standing project aiming to improve different aspects of the spectroscopy of this band by highly sensitive cavity ring down spectroscopy (CRDS).In the present contribution, low pressure (5 and 10 Torr) spectra of pure O2 were recorded with unprecedented sensitivity in the 7920–8085 cm−1 interval (noise equivalent absorption, αmin, on the order of 10−12 cm−1) using an external cavity diode laser. About 170 lines including electric quadrupole transitions were accurately measured. The weakest lines have intensity on the order of 10−30 cm/molecule. The coupling of the CRDS spectrometer with a self-referenced frequency comb allows for an important gain on the accuracy of the line center determination. Detailed line profile analysis using the quadratic speed-dependent Nelkin-Ghatak profile was performed for a series of twelve lines recorded for pressures up to 150 Torr. In particular, the very weak self-pressure shifts (on the order of 10−3 cm−1/atm) could be determined for the first time. Line intensities with uncertainty of 1% are reported for lines with intensity larger than 10−28 cm/molecule. Accurate spectroscopic parameters of the a1Δg(v = 0) upper level were fitted to the zero-pressure line centers. An rms value of 108 kHz (3.6 × 10−6 cm−1) is achieved for the (meas.-calc.) differences of the 16O2a1Δg(v = 0) upper level (Jmax= 37). Significant deviations compared to the HITRAN database are discussed.

Lineshape test on overlapped transitions (R9F1, R9F2) of the 2v3 band of 12CH4 by frequency-stabilized cavity ring-down spectroscopy

The performances of a multi-spectral fit for the spectra of pressure-broadened overlapping lines (R9F1, R9F2) of 12CH4 in binary mixtures with N2 were studied by applying different lineshape models, from the simplest Voigt profile (VP) to the Harmann-Tran profile (HTP). Line-mixing was approximated in the first order in the spectral fits. Data were acquired using a high-resolution cavity ring-down spectrometer of minimum detectable absorption coefficient of 2.8 × 10−12 cm−1. The lines were observed with a signal-to-noise ratio of 19 365 for pressures from 5 to 40 kPa. The study reveals that the multi-spectral fits using the HTP and the speed-dependent Nelkin–Ghatak profile (SDNGP) yield the best among all tested. The two models gave the maximum relative residuals of less than 0.065 %. All things considered, the HTP and the SDNGP appear to be the most reliable models for treating the present case of multi-spectral fitting of unresolved dual-component spectra.

Line shape parameters of helium-broadened 12C16O transitions in the 3 → 0 overtone band near 1.57 µm

Frequency-stabilized cavity ring-down spectroscopy was used to determine helium-broadened 12C16O line shape parameters for eleven lines in the 3 → 0 vibrational overtone band, including helium pressure broadening and pressure shifting. The spectra provided evidence of two non-Voigt mechanisms affecting line shape: collisional (Dicke) narrowing and speed dependence of the line broadening. Multispectrum fits of the speed dependent Nelkin-Ghatak profile (with the quadratic approximation for speed dependence) enabled precise determinations of the line shape parameters associated with these two mechanisms. The measured line shape parameters are presented and are compared to past work for helium-broadened 12C16O transitions in the 3 → 0 vibrational band (broadening coefficients) and in the 1 → 0 fundamental vibrational band (broadening and shifting coefficients). Significant reductions in the uncertainty of the broadening coefficients are demonstrated, along with the first determinations of helium pressure shifting and narrowing parameters for this transition. Analysis of the spectra at elevated pressure reveals evidence of line mixing with fitted first-order line mixing parameters that are consistent with literature values.

Cavity ring-down spectrometer for high-fidelity molecular absorption measurements

We present a cavity ring-down spectrometer which was developed for near-infrared measurements of laser absorption by atmospheric greenhouse gases. This system has several important attributes that make it possible to conduct broad spectral surveys and to determine line-by-line parameters with wide dynamic range, and high spectral resolution, sensitivity and accuracy. We demonstrate a noise-equivalent absorption coefficient of 4×10−12cm−1Hz−1/2 and a signal-to-noise ratio of 1.5×106:1 in an absorption spectrum of carbon monoxide. We also present high-resolution measurements of trace methane in air spanning more than 1.2THz and having a frequency axing with an uncertainty less than 100kHz. Finally, we discuss how this system enables stringent tests of advanced line shape models. To illustrate, we measured an air-broadened carbon dioxide transition over a wide pressure range and analyzed these data with a multi-spectrum fit of the partially correlated, quadratic speed-dependent Nelkin–Ghatak profile. We obtained a quality-of-fit parameter in the multispectrum fit equal to 36,000, thus quantifying small-but-measurable limitations of the model profile. This analysis showed that the line shape depends upon collisional narrowing, speed dependent effects and partial correlations between velocity- and phase-changing collisions.

Observations of Dicke narrowing and speed dependence in air-broadened CO₂ lineshapes near 2.06 μm.

Frequency-stabilized cavity ring-down spectroscopy was used to study CO2 lineshapes in the (20013) ← (00001) band centered near 2.06 μm. Two rovibrational transitions were chosen for this study to measure non-Voigt collisional effects for air-broadened lines over the pressure range of 7 kPa-28 kPa. Lineshape analysis for both lines revealed evidence of simultaneous Dicke (collisional) narrowing and speed-dependent effects that would introduce biases exceeding 2% in the retrieved air-broadening parameters if not incorporated in the modeling of CO2 lineshapes. Additionally, correlations between velocity- and phase/state changing collisions greatly reduced the observed Dicke narrowing effect. As a result, it was concluded that the most appropriate line profile for modeling CO2 lineshapes near 2.06 μm was the correlated speed-dependent Nelkin-Ghatak profile, which includes all of the physical effects mentioned above and leads to a consistent set of line shape parameters that are linear with gas pressure.

Low-pressure line-shape study in molecular oxygen with absolute frequency reference

We present a line-shape analysis of the rovibronic R1 Q2 transition of the oxygen B band resolved by the Pound-Drever-Hall-locked frequency-stabilized cavity ring-down spectroscopy technique in the low pressure range. The frequency axis of the spectra is linked by the ultra-narrow diode laser to the optical frequency comb in order to measure the absolute frequency at each point of the recorded spectra. Experimental spectra are fitted with various line-shape models: the Voigt profile, the Galatry profile, the Nelkin-Ghatak profile, the speed-dependent Voigt profile, and the speed-dependent Nelkin-Ghatak profile with quadratic and hypergeometric approximations for the speed dependence of collisional broadening and shifting. The influences of Dicke narrowing, speed-dependent effects, and correlation between phase- and velocity-changing collisions on the line shape are investigated. Values of line-shape parameters, including the absolute frequency of the transition 435685.24828(46) GHz, are reported.

Transition frequencies of oxygen B-band lines measured with optical frequency comb assisted cavity ring-down spectroscopy

Transition frequencies of weak B-band lines of O2 were measured with low uncertainties. These results were compared to data available in the literature. In order to achieve reported accuracy we took into account in the data analysis several subtle line-shape effects, such as speed dependence of collisional broadening and Dicke narrowing. We found that the speed-dependent Nelkin-Ghatak profile can properly describe experimental line shapes in the investigated pressure range of up to 22 Torr. Our experimental setup consists of the Pound-Drever-Hall locked frequency-stabilized cavity ring-down spectrometer. This spectrometer provides high-resolution and very high signal-to-noise ratio of measured spectra. It was recently linked to the optical frequency comb working in the visible region of spectrum. This upgrade allows for accurate determination of the absolute frequency axis of the CRD spectrum by direct measurement of the probe laser frequency.