Prediction of high-order line-shape parameters for air-broadened O2 lines using requantized classical molecular dynamics simulations and comparison with measurements

Abstract

et al.. Prediction of high-order line-shape parameters for air-broadened O2 lines using requantized classical molecular dynamics simulations and comparison with measurements. Journal of Quantitative Spectroscopy and Radiative Transfer, Elsevier, 2019, 222-223, pp. AbstractLine-shape models such as the Hartmann-Tran (HT) profile have adjustable high-order parameters that are usually determined by fits to experimental spectra. As an alternative approach, we demonstrate that fitting the HT profile to theoretical spectra provides high-order line-shape parameters for O2 transitions that are consistent with experimentally determined values. To this end, normalized absorption spectra of air-broadened O2 lines were computed without adjustable parameters using requantized classical molecular dynamics simulations (rCMDS). These theoretical calculations were made at a pressure of 203 kPa and for values of the Doppler width that cover near-Doppler-limited to collisional-broadened pressure conditions. Hartmann-Tran (HT) line profiles with adjustable line-shape parameters were then simultaneously fit to the set of rCMDS-calculated spectra in a global multispectrum analysis. The retrieved high-order line-shape parameters (i.e. the speed dependence of the line broadening and the Dicke narrowing coefficient) were subsequently used as fixed HT parameters in the analysis of seven air-broadened O2 lines of the band. The spectra were measured over a fifteen-fold range of total gas pressure at high spectral resolution and signal-to-noise ratio with a frequency-stabilized cavity ring-down spectroscopy system. We show that these predicted parameters enable all the measured lines to be fit to within 1 %, which is much better than best fits of the Voigt line profile to the measured spectra. This approach opens the route for predicting high-order line-shape parameters from first-principles calculations and for their inclusion in spectroscopic databases. Furthermore, the temperature dependences of the broadening coefficient and its speed dependent component for air-broadened O2 lines were also calculated using rCMDS.

Prediction of line shape parameters and their temperature dependences for CO2–N2 using molecular dynamics simulations

“…As shown in Refs. [46,54,55] and later in this paper, if we can predict all the higher-order lineshape parameters (e.g. , or ), the value of can be easily and accurately determined using measured spectra.…”

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

“…[19]) and O2 in air (e.g. [17]), respectively. The molecules were considered as rigid rotors and all the effects of vibrational motion were disregarded in the calculations.…”

“…In these works, line-shape parameters were generally retrieved from high quality spectra measured under various pressure conditions, using a multi-spectrum fitting procedure [15]. In parallel, it was recently shown that line-shape parameters for linear molecules can be predicted using requantized classical molecular dynamics simulations (rCMDS) [16,17]. Indeed, in these rCMDS, one can simulate the time evolution of a very large number of molecules from an intermolecular potential.…”

“…The Fourier-Laplace transform of this auto-correlation function directly yields the corresponding absorption spectrum. It was demonstrated in a series of papers that rCMDS are capable to reproduce various collisional effects affecting the absorption spectrum of different molecular systems [16][17][18][19][20][21][22][23][24][25][26][27]. In a recently published paper [16], rCMDS were used to predict line-shape parameters for N2-broadened CO2 lines.…”

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

“…Only Voigt line-shape parameters are provided by HITRAN2016 and GEISA2015. Recently, some studies showed that the Voigt profile leads to significant deviations from measured spectra for the 1.27 m band lines [5,[20][21][22] and that the speed dependence of the collisional broadening and the collision-induced velocity changes have to be taken into account. Using the quadratic speeddependent Voigt (qsdV) profile to fit high sensitivity CRDS air-broadened spectra of 97 O2 lines in the 1.27 m band, Mendonca et al [5] obtained fit residuals largely improved with respect to the Voigt profile.…”