Subpromille Measurements and Calculations of CO (3-0) Overtone Line Intensities.

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‰ 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‰ level. This set of highly accurate intensities can provide an intrinsic reference for reducing biases in future measurements of spectroscopic peak areas.

Assessment of the precision, bias and numerical correlation of fitted parameters obtained by multi-spectrum fits of the Hartmann-Tran line profile to simulated absorption spectra.

Although the Voigt profile has long been used to analyze absorption spectra, the quest for increased precision, accuracy and generality drives the application of advanced models of atomic and molecular line shapes. To this end, the Hartmann-Tran profile is now recommended as a standard for high-resolution spectroscopy because it parameterizes relevant higher-order physical effects, is computationally efficient, and reduces to other widely used profiles as limiting cases. This work explores the uncertainty with which line shape parameters can be obtained from constrained multi-spectrum fits of spectra simulated with this standard profile, varying uncertainty levels in the spectrum detuning and absorption axes, and spanning a range of sampling density, pressure, and line shape parameter values. The analysis focuses on how noise-limited measurement precision of frequency detuning and absorption drive statistical uncertainties in fitted parameters and numerical correlations between these quantities. Also, we quantify the degree of equivalence between the full Hartmann-Tran profile and those derived from it in terms of fitted peak areas and line shape parameters. Finally, we introduce a new open-source software package named Multi-spectrum Analysis Tool for Spectroscopy (MATS), which allows users to fit the HTP and its derived profiles to experimental or simulated absorption spectra to explore the limits of the HTP under actual experimental or user-defined conditions.

Near-infrared cavity ring-down spectroscopy measurements of nitrous oxide in the (4200)←(0000) and (5000)←(0000) bands.

Using frequency-agile rapid scanning cavity ring-down spectroscopy, we measured line intensities and line shape parameters of 14N2 16O 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.

Air-broadening in near-infrared carbon dioxide line shapes: Quantifying contributions from O2, N2, and Ar.

We measured air broadening in the (30012) ← (00001) carbon dioxide (CO2) band up to J″=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.

Improvement of the spectroscopic parameters of the air- and self-broadened N2O and CO lines for the HITRAN2020 database applications.

This paper outlines the major updates of the line-shape parameters that were performed for the nitrous oxide (N2O) and carbon monoxide (CO) molecules listed in the HITRAN2020 database. We reviewed the collected measurements for the air- and self-broadened N2O 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 N2O 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 N2O 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 online.

Cavity ring-down spectroscopy of CO2 near λ = 2.06 µm: Accurate transition intensities for the Orbiting Carbon Observatory-2 (OCO-2) "strong band".

The λ = 2.06 µm absorption band of CO2 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 12C16O2 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 %.

Twenty-Five-Fold Reduction in Measurement Uncertainty for a Molecular Line Intensity.

To accurately attribute sources and sinks of molecules like CO_{2}, remote sensing missions require line intensities (S) with relative uncertainties u_{r}(S)<0.1%. However, discrepancies in S of ≈1% are common when comparing different experiments, thus limiting their potential impact. Here we report a cavity ring-down spectroscopy multi-instrument comparison which revealed that the hardware used to digitize analog ring-down signals caused variability in spectral integrals which yield S. Our refined approach improved measurement accuracy 25-fold, resulting in u_{r}(S)=0.06%.

Towards a taxonomy of topology for polynuclear aromatic hydrocarbons: linking electronic and molecular structure.

Trends linking the topological characteristics of polynuclear aromatic hydrocarbons (PAH) to their electronic properties are reported. TD-DFT electronic spectra computations, using the 6-31G* basis set and B3LYP exchange correlation functional, were calculated for a series of PAH, allowing for the HOMO-LUMO gaps to be reported. Clar structures provide an avenue to link the physical structure and the aromaticity of the molecule; which, when extended by bond length and harmonic oscillator model of aromaticity analysis, provide powerful tools to understand the link between electronic and physical structure. These results lead to the conclusion that all PAH structures show a decrease in HOMO-LUMO gap as a function of size, but the rate of that decrease is directly related to the topology of the molecules. A PAH taxonomy was developed that categorizes PAH into categories with similar topological properties, which allows for modelling of changes in the HOMO-LUMO gap with PAH size. An atom-pair minimization algorithm was used to calculate the binding energy (BE) of homogeneous dimers of the studied PAH. The BE per carbon atom increases with the overall size of the structure to an asymptotic limit, but as with the HOMO-LUMO gap, topology plays a critical secondary factor. Previously published, experimentally determined optical band gaps (OBG) from Tauc/Davis-Mott analysis of extinction spectra in various laminar, non-premixed flames produced a correlation between the HOMO-LUMO gaps of high-symmetry, nearly circular D2h symmetry molecules to molecular size. The work presented here provides a much more nuanced and predictive evaluation of how OBG depends on structure and size.

Extinction measurements for optical band gap determination of soot in a series of nitrogen-diluted ethylene/air non-premixed flames.

Visible light extinction was measured in a series of nitrogen-diluted, ethylene/air, non-premixed flames and this data was used to determine the optical band gap, OBG, as a function of flame position. Collimated light from a supercontinuum source is telescopically expanded and refocused to match the f- number of a dispersing monochromator. The dispersed light is split into a power metering channel and a channel that is periscoped and focused into the flame. The transmitted light is then recollimated and focussed onto a silicon photodiode detector. After tomographic reconstruction of the radial extinction field, the OBG was derived from the near-edge absorption feature using Tauc/Davis-Mott analysis. A slight evolution in OBG was observed throughout all flame systems with a consistent range of OBG observed between approximately 1.85 eV and 2.35 eV. Averaging over all positions the mean OBG was approximately 2.09 eV for all flame systems. Comparing these results to previously published computational results relating calculated HOMO-LUMO gaps for a variety of D2h PAH molecules to the number of aromatic rings in the structure, showed that the observed optical band gap is consistent with a PAH of about 14 rings or a conjugation length of 0.97 nm. This work provides experimental support to the model of soot formation where the transition from chemical to physical growth starts at a modest molecular size; about the size of circumpyrene.