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

Abstract

We show in this paper that requantized classical molecular dynamics simulations (rCMDSs) are capable of predicting various refined spectral-shape parameters of absorption lines of CO2 broadened by N2 with high precision. Combining CMDSs and a requantization procedure, we computed the auto-correlation function of the CO2 dipole moment responsible for the absorption transition. Its Fourier-Laplace transform directly yields the spectrum. Calculations were made for two temperatures, 200 and 296 K, at 1 atm and for a large range of Doppler widths, from the near-Doppler to the collision-dominant regimes. For each temperature and each line, the spectra calculated for various Doppler widths were simultaneously fit with the Hartmann-Tran (HT) profile. This refined profile, which takes into account the effects of the speed dependent collisional line broadening, the Dicke narrowing, and the collisional line mixing, was recommended as a reference model to be used in high-resolution spectroscopy (instead of the simplified Voigt model). The HT parameters retrieved from the rCMDS-calculated spectra were then directly compared with those deduced from high-precision measurements [J. S. Wilzewski et al., J. Quant. Spectrosc. Radiat. Transfer 206, 296–305 (2018)]. The results show a very good agreement, even for those parameters whose influence on the spectra is very small. Good agreement is also obtained between measured and predicted temperature dependences of these parameters. This demonstrates that rCMDS is an excellent tool, highly competitive with respect to high quality measurements for precise line-shape studies.

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

“…Only CO2-O2 interactions being taken into account, this is equivalent to CO2 infinitely diluted in 0.5 atm of O2. Details on the calculations method can be found in [16] and references therein. A site-site functional form with Coulombic and atom-atom contributions was used to describe the interaction potential between CO2 and O2 molecules.…”

“…To this aim, the rCMDS-calculated spectra of CO2 in N2 were fitted with the speed-dependent Nelkin-Ghatak (sdNG) profile in order to retrieve the collisional line broadening coefficient, its speed dependence, the Dicke narrowing and the first-order linemixing parameters. The predicted parameters were then compared with those determined from high quality measurements [14] showing very good agreement, even for the high-order lineshape parameters [16]. rCMDS are thus a complementary tool for line-shape studies provided that an accurate intermolecular potential is available.…”

“…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.…”

“…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. To this aim, the rCMDS-calculated spectra of CO2 in N2 were fitted with the speed-dependent Nelkin-Ghatak (sdNG) profile in order to retrieve the collisional line broadening coefficient, its speed dependence, the Dicke narrowing and the first-order linemixing parameters.…”

“…Furthermore, the calculations can be performed relatively easily for various temperature conditions, which is not always possible for laboratory measurements. As a follow up to the study of [16], in this paper, rCMDS are used to predict line-shape parameters for O2-broadened CO2 lines.…”