Search

We present a primary method for absolute reference material-free measurement of gaseous elemental mercury mass concentration based on laser absorption spectroscopy at wavelengths near 254 nm and anchored to the international system of units. A 76 m-long multipass cell provides a detection limit of 1 ng m

Intensities of lines in the near-infrared second overtone band (3--0) of $^{12}$C$^{16}$O are measured and calculated to an unprecedented degree of precision and accuracy. Agreement between theory and experiment to better than 1 $\permil$ is demonstrated by results from two laboratories involving two independent absorption- and dispersion-based cavity-enhanced techniques. Similarly, independent Fourier transform spectroscopy measurements of stronger lines in this band yield mutual agreement and consistency with theory at the 1 $\permil$ level. This set of highly accurate intensities can provide an intrinsic reference for reducing biases in future measurements of spectroscopic peak areas., Comment: main paper plus supporting material

Measurements of isotope ratios are predominantly made with reference to standard specimens that have been characterized in the past. In the 1950s, the carbon isotope ratio was referenced to a belemnite sample collected by Heinz Lowenstam and Harold Urey1 in South Carolina’s PeeDee region. Due to exhaustion of the sample since then, reference materials that are traceable to the original artefact are used to define the Vienna PeeDee Belemnite scale for stable carbon isotope analysis2. However, these reference materials have also become exhausted or proven to exhibit unstable composition over time3, mirroring issues with the international prototype of the kilogram that led to a revised International System of Units4. A campaign to elucidate the stable carbon isotope ratio of Vienna PeeDee Belemnite is underway5, but independent measurement techniques are required to support it. Here we report an accurate value for the stable carbon isotope ratio inferred from infrared absorption spectroscopy, fulfilling the promise of this fundamentally accurate approach6. Our results agree with a value recently derived from mass spectrometry5 and therefore advance the prospects of International System of Units–traceable isotope analysis. Further, our calibration-free method could improve mass balance calculations and enhance isotopic tracer studies in carbon dioxide source apportionment. Isotope ratio measurements are complicated by the instabilities of composition in reference samples. Now a calibration-free method relying on infrared spectroscopy provides measurements that are traceable to International System of Units standards.

We present a direct comparison between two independent methods for the measurement of gaseous elemental mercury (GEM) mass concentration: isotope dilution cold-vapor inductively coupled plasma mass spectrometry (ID-CV-ICP-MS) and laser absorption spectroscopy (LAS). The former technique combined with passive sorbent tube sampling is currently the primary method at NIST for mercury gas standards traceability to the International System of Units (SI). This traceability is achieved via measurements on a mercury-containing reference material. The latter technique has been recently developed at NIST and involves real-time measurements of light attenuation caused by GEM, with SI traceability based in part on the known spontaneous emission lifetime of the probed 6 (1)S(0) ─ 6 (3)P(1) inter-combination transition of elemental mercury (Hg(0)). Using a steady-flow Hg(0)-in-air generator to produce samples measured by both methods, we use LAS to measure the sample gas and in parallel we collect the Hg(0) on sorbent tubes to be subsequently analyzed using ID-CV-ICP-MS. Over the examined mass concentration range (41 μg/m(3) to 287 μg/m(3) Hg(0) in air) the relative disagreement between the two approaches ranged from 1.0% to 1.8%. The relative combined standard uncertainty on average is 0.4% and 0.9%, for the LAS and MS methods, respectively. Our comparison studies help validate the accuracy of the ID-CV-ICP-MS primary method as well as establish the LAS technique as an attractive alternative primary method for SI-traceable measurements of GEM.

We present transition intensity measurements for the 20011 ← 00001 band of carbon dioxide to test quantum theory and support sensing and isotopic analysis. Preliminarily, a relative combined standard uncertainty of ≤ 0.2 % was achieved.

This paper describes the updates of the line positions and intensities for the carbon dioxide transitions in the 2020 edition of the HITRAN spectroscopic database. The new line list for all 12 naturally abundant isotopologues of carbon dioxide replaces the previous one from the HITRAN2016 edition. This update is primarily motivated by several issues related to deficient HITRAN2016 line positions and intensities that have been identified from laboratory and atmospheric spectra. Critical validation tests for the spectroscopic data were carried out to find problems caused by inaccuracies in CO2 line parameters. New sources of data were selected for the bands that were deemed problematic in the HITRAN2016 edition. Extra care was taken to retain the consistency in the data sources within the bands. The comparisons with the existing theoretical and semi-empirical databases (including ExoMol, NASA Ames, and CDSD-296) and with available experimental works were carried out. The HITRAN2020 database has been extended by including additional CO2 bands above 8000 cm−1, and magnetic dipole lines of CO2 were introduced in HITRAN for the first time by including the ν2+ν3 band in the 3.3-µm region. Although the main topic of this article is line positions and intensities, for consistency a recent algorithm for the line-shape parameters proposed in Hashemi et al. JQSRT (2020) was reapplied (after minor revisions) to the line list.

High-resolution, laboratory, absorption spectra of the a 1 Δ g ← X 3 Σ g - oxygen ( O2 ) band measured using cavity ring-down spectroscopy were fitted using the Voigt and speed-dependent Voigt line shapes. We found that the speed-dependent Voigt line shape was better able to model the measured absorption coefficients than the Voigt line shape. We used these line shape models to calculate absorption coefficients to retrieve atmospheric total columns abundances of O2 from ground-based spectra from four Fourier transform spectrometers that are a part of the Total Carbon Column Observing Network (TCCON). Lower O2 total columns were retrieved with the speed-dependent Voigt line shape, and the difference between the total columns retrieved using the Voigt and speed-dependent Voigt line shapes increased as a function of solar zenith angle. Previous work has shown that carbon dioxide ( CO2 ) total columns are better retrieved using a speed-dependent Voigt line shape with line mixing. The column-averaged dry-air mole fraction of CO2 ( XCO2 ) was calculated using the ratio between the columns of CO2 and O2 retrieved (from the same spectra) with both line shapes from measurements taken over a 1-year period at the four sites. The inclusion of speed dependence in the O2 retrievals significantly reduces the air mass dependence of XCO2 , and the bias between the TCCON measurements and calibrated integrated aircraft profile measurements was reduced from 1 % to 0.4 %. These results suggest that speed dependence should be included in the forward model when fitting near-infrared CO2 and O2 spectra to improve the accuracy of XCO2 measurements.

Using frequency-agile rapid scanning cavity ring-down spectroscopy, we measured line intensities and line shape parameters of 14N216O in air in the (4200)←(0000) and (5000)←(0000) bands near 1.6 μm. The absorption spectra were modeled with multi-spectrum fits of Voigt and speed-dependent Voigt profiles. The measured line intensities and air-broadening parameters exhibit deviations of several percent relative to values provided in HITRAN 2016. Our measured intensities for these two bands have relative combined standard uncertainties of ~1% which is approximately five times smaller than literature values. Comparison of the present air-broadening and speed-dependent broadening parameters to experimental literature values for other rotation-vibration bands of N2O indicates significant differences in magnitude and J-dependence. For applications requiring high spectral fidelity, these results suggest that the assumption of band-independent line shape parameters is not appropriate.

Measurements of ultrahigh-fidelity absorption spectra can help validate quantum theory, engineer ultracold chemistry, and remotely sense atmospheres. Recent achievements in cavity-enhanced spectroscopy using either frequency-based dispersion or time-based absorption approaches have set new records for accuracy with uncertainties at the sub-per-mil level. However, laser scanning or susceptibility to nonlinearities limits their ultimate performance. Here we present cavity buildup dispersion spectroscopy (CBDS), probing the CO molecule as an example, in which the dispersive frequency shift of a cavity resonance is encoded in the cavity’s transient response to a phase-locked non-resonant laser excitation. Beating between optical frequencies during buildup exactly localizes detuning from mode center, and thus enables single-shot dispersion measurements. CBDS can yield an accuracy limited by the chosen frequency standard and measurement duration and is currently 50 times less susceptible to detection nonlinearity compared to intensity-based methods. Moreover, CBDS is significantly faster than previous frequency-based cavity-enhanced methods. The generality of CBDS shows promise for improving fundamental research into a variety of light–matter interactions. Cavity-enhanced spectroscopy is used to analyse light–matter interactions in fields such as ultracold chemistry and planetary science but measurement performance can be hampered by nonlinearities and long acquisition times. Here, the authors report a technique called cavity build up dispersion spectroscopy to measure dispersive frequency shifts demonstrating increased acquisition speeds and less susceptibility to detector nonlinearity.

We demonstrate Doppler-free two-photon cavity ring-down spectroscopy using a narrow linewidth quantum cascade laser locked to a high-finesse cavity. Two-photon absorption of 14N216O at ~0 = 2207.507 cm−1 is reported.

The accuracy of atmospheric trace gas retrievals depends directly on the accuracy of the molecular absorption model used within the retrieval algorithm. For remote sensing of well-mixed gases, such as carbon dioxide (CO2), where the atmospheric variability is small compared to the background, the quality of the molecular absorption model is key. Recent updates to oxygen (O2) absorption coefficients (ABSCO) for the 0.76 µm A-band and the water vapor (H2O) continuum model within the 1.6 µm and 2.06 µm CO2 bands used within the Orbiting Carbon Observatory (OCO-2 and OCO-3) algorithm are described here. Updates in the O2 A-band involve the inclusion of new laboratory measurements within multispectrum fits to improve relative consistency between O2 line shapes and collision-induced absorption (CIA). The H2O continuum model has been updated to MT _ CKD v3.2, which has benefited from information from a range of laboratory studies relative to the model utilized in the previous ABSCO version. Impacts of these spectroscopy updates have been evaluated against ground-based atmospheric spectra from the Total Carbon Column Observing Network (TCCON) and within the framework of the OCO-2 algorithm, using OCO-2 soundings covering a range of atmospheric and surface conditions. The updated absorption coefficients (ABSCO version 5.1) are found to offer improved fitting residuals and reduced biases in retrieved surface pressure relative to the previous version (ABSCO v5.0) used within B8 and B9 of the OCO-2 retrieval algorithm and have been adopted for the OCO B10 Level 2 algorithm.

The λ = 2.06 μm absorption band of CO(2) is widely used for the remote sensing of atmospheric carbon dioxide, making it relevant to many important top-down measurements of carbon flux. The forward models used in the retrieval algorithms employed in these measurements require increasingly accurate line intensity and line shape data from which absorption cross-sections can be computed. To overcome accuracy limitations of existing line lists, we used frequency-stabilized cavity ring-down spectroscopy to measure 39 transitions in the (12)C(16)O(2) absorption band. The line intensities were measured with an estimated relative combined standard uncertainty of u(r) = 0.08 %. We predicted the J-dependence of the measured intensities using two theoretical models: a one-dimensional spectroscopic model with Herman-Wallis rotation-vibration corrections, and a line-by-line ab initio dipole moment surface model [Zak et al. JQSRT 2016;177:31–42]. For the second approach, we fit only a single factor to rescale the theoretical integrated band intensity to be consistent with the measured intensities. We find that the latter approach yields an equally adequate representation of the fitted J-dependent intensity data and provides the most physically general representation of the results. Our recommended value for the integrated band intensity equal to 7.183 × 10(−21) cm molecule(−1) ± 6 × 10(−24) cm molecule(−1) is based on the rescaled ab initio model and corresponds to a fitted scale factor of 1.0069 ± 0.0002. Comparisons of literature intensity values to our results reveal systematic deviations ranging from −1.16 % to +0.33 %.

We report Doppler-free two-photon absorption of N(2)O at λ = 4.53 μm, measured by cavity ring-down spectroscopy. High power was achieved by optical self-locking of a quantum cascade laser to a linear resonator of finesse [Formula: see text] , and accurate laser detuning over a 400 MHz range was measured relative to an optical frequency comb. At a sample pressure of p = 0.13 kPa, we report a large two-photon cross-section per molecule of [Formula: see text] cm(4) s for the Q(18) rovibrational transition at a resonant frequency of ν(0) = 66179400.8 MHz.

We report Doppler-free two-photon absorption of ${\mathrm{N}}_{2}\mathrm{O}$ at $\ensuremath{\lambda}=4.53\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$, measured by cavity ring-down spectroscopy. High power was achieved by optical self-locking of a quantum cascade laser to a linear resonator of finesse $\mathcal{F}=22730$, and accurate laser detuning over a 400-MHz range was measured relative to an optical frequency comb. At a sample pressure of $p=0.13\phantom{\rule{0.16em}{0ex}}\mathrm{kPa}$, we report a large two-photon cross section per molecule of ${\ensuremath{\sigma}}_{13}^{(2)}=8.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}41}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{4}\phantom{\rule{0.16em}{0ex}}\mathrm{s}$ for the $Q(18)$ rovibrational transition at a resonant frequency of ${\ensuremath{\nu}}_{0}=66179400.8\phantom{\rule{0.16em}{0ex}}\mathrm{MHz}$.

The $\lambda$ = 2.06 $\mu$m absorption band of CO$_2$ is widely used for the remote sensing of atmospheric carbon dioxide, making it relevant to many important top-down measurements of carbon flux. The forward models used in the retrieval algorithms employed in these measurements require increasingly accurate line intensity and line shape data from which absorption cross-sections can be computed. To overcome accuracy limitations of existing line lists, we used frequency-stabilized cavity ring-down spectroscopy to measure 39 transitions in the $^{12}$C$^{16}$O$_2$ absorption band. The line intensities were measured with an estimated relative combined standard uncertainty of $u_r$ = 0.08 %. We predict the $J$-dependence of the measured intensities using two theoretical modesl: a one-dimensional spectroscopic model with Herman-Wallis rotation-vibration corrections, and a line-by-line ab initio dipole moment surface model [Zak et al. JQSRT 2016;177:31-42]. For the second approach, we fit only a single factor to rescale the theoretical integrated band intensity to be consistent with the measured intensities. We find that the latter approach yields an equally adequate representation of the fitted $J$-dependent intensity data and provides the most physically general representation of the results. Our recommended value for the integrated band intensity equal to 7.183$\times$10$^{-21}$ cm molecule$^{-1}$ $\pm$ 6$\times$10$^{-24}$ cm molecule$^{-1}$ is based on the rescaled ab initio model and corresponds to a fitted scale factor of 1.0069 $\pm$ 0.0002. Comparisons of literature intensity values to our results reveal systematic deviations ranging from $-$1.16 % to +0.33 %., Comment: 34 pages, 10 figures, 4 tables, 54 references

We describe highly accurate molecular line positions determined by a recently developed comb-locked cavity ring-down spectrometer. Molecular transition frequencies near 1.6 pm are determined with relative total uncertainties below 1 x 10-12 (200 Hz absolute uncertainty).

International audience; We review progress, since publication of the book "Collisional effects on molecular spectra Laboratory experiments and models, consequences for applications" (Elsevier, Amsterdam, 2008), on measuring, modeling and predicting the influence of pressure (ie of intermolecular collisions) on the spectra of gas molecules. We first introduce recently developed experimental techniques of high accuracy and sensitivity. We then complement the aforementioned book by presenting the theoretical approaches, results and data proposed (mostly) in the last decade on the topics of isolated line shapes, line-broadening and-shifting, line-mixing, the far wings and associated continua, and collision-induced absorption. Examples of recently demonstrated consequences of the progress in the description of spectral shapes for some practical applications (metrology, probing of gas media, climate predictions) are then given. Remaining issues and directions for future research are finally discussed.

Isotope dilution-cold-vapor-inductively coupled plasma mass spectrometry (ID-CV-ICPMS) has become the primary standard for measurement of gaseous elemental mercury (GEM) mass concentration. However, quantitative mass spectrometry is challenging for several reasons including (1) the need for isotopic spiking with a standard reference material, (2) the requirement for bias-free passive sampling protocols, (3) the need for stable mass spectrometry interface design, and (4) the time and cost involved for gas sampling, sample processing, and instrument calibration. Here, we introduce a high-resolution laser absorption spectroscopy method that eliminates the need for sample-specific calibration standards or detailed analysis of sample treatment losses. This technique involves a tunable, single-frequency laser absorption spectrometer that measures isotopically resolved spectra of elemental mercury (Hg) spectra of 6 1S0 ← 6 3P1 intercombination transition near λ = 253.7 nm. Measured spectra are accurately modeled...

There are many gas phase compounds present in the atmosphere that affect and influence the earth's climate. These compounds absorb and emit radiation, a process which is the fundamental cause of the greenhouse effect. The major greenhouse gases in the earth's atmosphere are carbon dioxide, methane, nitrous oxide, and ozone. Some halocarbons are also strong greenhouse gases and are linked to stratospheric ozone depletion. Hydrocarbons and monoterpenes are precursors and contributors to atmospheric photochemical processes, which lead to the formation of particulates and secondary photo-oxidants such as ozone, leading to photochemical smog. Reactive gases such as nitric oxide and sulfur dioxide are also compounds found in the atmosphere and generally lead to the formation of other oxides. These compounds can be oxidized in the air to acidic and corrosive gases and contribute to photochemical smog. Measurements of these compounds in the atmosphere have been ongoing for decades to track growth rates and assist in curbing emissions of these compounds into the atmosphere. To accurately establish mole fraction trends and assess the role of these gas phase compounds in atmospheric chemistry, it is essential to have good calibration standards. The National Institute of Standards and Technology has been developing standards of many of these compounds for over 40 years. This paper discusses the development of these standards.

Reported here are highly accurate, experimentally measured ro-vibrational transition intensities for the R-branch of the (20012) - (00001) 12C16O2 band near λ = 2 μm. Measurements were performed by a frequency-stabilized cavity ring-down spectroscopy (FS-CRDS) instrument designed to achieve precision molecular spectroscopy in this important region of the infrared. Through careful control and traceable characterization of CO2 sample conditions, and through high-fidelity measurements spanning several months in time, we achieve relative standard uncertainties for the reported transition intensities between 0.15 % and 0.46 %. Such high accuracy spectroscopy is shown to provide a stringent test of calculated potential energy and ab initio dipole moment surfaces, and therefore transition intensities calculated from first principles.

We report an important correction to recently reported 12C16O2 transition frequencies and derived spectroscopic constants. A total of 44 previously reported frequencies corresponding to transitions in the (30012 ← 00001) and (30013 ← 00001) band with uncertainties as low as 200 Hz are modified by including the effect of photon-absorption-induced recoil shifts. This shifting mechanism decreases all the transition frequencies by approximately 1.7 kHz.

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.

This paper outlines the major updates of the line-shape parameters that were performed for the nitrous oxide (N 2 O) and carbon monoxide (CO) molecules listed in the HITRAN2020 database. We reviewed the collected measurements for the air- and self-broadened N 2 O and CO spectra to determine proper values for the spectroscopic parameters. Careful comparisons of broadening parameters using the Voigt and speed-dependent Voigt line-shape profiles were performed among various published results for both N 2 O and CO. Selected data allowed for developing semi-empirical models, which were used to extrapolate/interpolate existing data to update broadening parameters of all the lines of these molecules in the HITRAN database. In addition to the line broadening parameters (and their temperature dependences), the pressure shift values were revised for N 2 O and CO broadened by air and self for all the bands. The air and self speed-dependence of the broadening parameter for these two molecules were added for every transition as well. Furthermore, we determined the first-order line-mixing parameters using the Exponential Power Gap (EPG) scaling law. These new parameters are now available at HITRAN o n l i n e .

We present measurements of molecular transition frequencies based on the comb-locked cavity ring-down spectroscopy technique, reporting vacuum transition frequencies of Doppler-broadened 12C16O2 in the 1.6 μm region for the (30012) ← (00001) and (30013) ← (00001) bands with an average combined standard uncertainty of ≈ 1 kHz. A global four-state model was fit to these data and literature values to provide spectroscopic parameters and a best-case fit precision of 4 kHz. We identified and quantified an interaction between the (30012) and (33301) states which was manifest as an observed Fermi-resonance-type perturbation in the (30012) ← (00001) band. This interaction was accounted for in the global fit, substantially reducing systematic uncertainties in the spectroscopic parameters of the (30012) state and altering many predicted transition frequencies in the (30012) ← (00001) band by 1 MHz or more relative to literature values. The accurate transition frequencies reported here may be considered as SI-traceable reference data for diverse applications including remote sensing and telecommunications.

We present broadband spectroscopic parameters in the a 1 Δ g − X 3 Σ g − (0,0) band of 16O2 centered near 1.27 µm. High-fidelity absorption spectra were acquired on room-temperature air samples over the pressure range of 3.3 kPa to 100 kPa using a length-stabilized cavity ring-down spectrometer, with spectrum frequencies linked to a Cs clock via an optical frequency comb. Parameters were determined by multi-spectrum fitting of advanced line profiles to the measured spectra. This analysis yielded line intensities, positions, collisional broadening and shifting coefficients, Dicke narrowing and speed dependent line shape parameters, first-order line mixing coefficients, and O2-air binary absorption coefficients for collision-induced absorption. We report relative standard uncertainties in the line intensities and broadening coefficients of 0.16 % and 0.05 %, respectively, and line position uncertainties of 60 kHz (2 × 10−6 cm−1). Our analysis reveals strong numerical correlation in fits of the Dicke narrowing and speed dependent widths and shows that inclusion of a recently proposed modification to the Hartmann Tran line profile can increase fitted Dicke narrowing frequencies by as much as 50 %.

We measured air broadening in the (30012) ← (00001) carbon dioxide (CO2) band up to Jʺ = 50 using frequency-agile rapid scanning cavity ring-down spectroscopy. By using synthetic air samples with varying levels of nitrogen, oxygen, and argon, multi-spectrum fitting allowed for the collisional broadening terms of each major air component to be simultaneously determined in addition to advanced line shape parameters at atmospherically relevant CO2 mixing ratios. These values were compared to broadener-specific line shape parameters from the literature. Fits to measured spectra were also constrained with results from requantized classical molecular dynamic simulations. We show that this approach enables differentiation between narrowing mechanisms in advanced line shape parameters retrieved from experimental spectra of limited signal-to-noise ratio.

Frequency-stabilized mid-infrared lasers are valuable tools for precision molecular spectroscopy. However, their implementation remains limited by complicated stabilization schemes. Here we achieve optical self-locking of a quantum cascade laser to the resonant leak-out field of a highly mode-matched two-mirror cavity. The result is a simple approach to achieving ultra-pure frequencies from high-powered mid-infrared lasers. For short time scales (, Comment: 4 pages, 5 figures, 31 references

High-precision measurements of radiocarbon (14C ) near or below a fraction modern 14C of one (F14C ≤ 1) are challenging and costly. An accurate, ultra-sensitive linear absorption approach to detecting 14C would provide a simple and robust bench-top alternative to off-site accelerator mass spectrometry facilities. Here we report the quantitative measurement of 14C in gas-phase samples of CO2 with F14C < 1 using cavity ring-down spectroscopy in the linear absorption regime. Repeated analysis of CO2 derived from the combustion of either biogenic or petrogenic sources revealed a robust ability to differentiate samples with F14C < 1. With a combined uncertainty of 14C /12C = 130 fmol/mol (F14C = 0.11), initial performance of the calibration-free instrument is sufficient to investigate a variety of applications in radiocarbon measurement science including the study of biofuels and bioplastics, illicitly traded specimens, bomb dating, and atmospheric transport.

CRDS spectra of CH4 highly diluted in air, measured for various pressure and temperature conditions

International audience; A detailed review and analysis of literature values for the absorption cross-section of ozone at room temperature at the mercury-line wavelength (253.65 nm, air) is reported. Data from fourteen independent sets of measurements spanning the years 1959-2016 were considered. The present analysis is based upon a revised assessment of all Type A and Type B uncertainty components for each previously reported cross-section. A consensus value for the absorption cross-section of 1.1329(35) × 10 −17 cm 2 molecule −1 is recommended based on statistical analysis of the weighted data. This new cross-section value is 1.23% lower and its uncertainty sixfold smaller than the uncertainty of the conventionally accepted reference value reported by Hearn (1961 Proc. Phys. Soc. 78 932-40).