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

Abstract

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 firstorder line-mixing parameters were retrieved for lines with 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.

Air-broadened N2O line-shape parameters and their temperature dependences by requantized classical molecular dynamics simulations

“…To the best of our knowledge, there is no measured nor previous calculated result for the temperature dependence exponents of 𝜁 for air-broadened N2O lines. However, the evolution of 𝑛 with the rotational quantum number is very similar to that obtained from experiments and calculations for CO2 in N2 [21,42] and in air [22].…”

“…As shown in [21,22,24], line-shape parameters predicted by rCMDS can be of several percent different from accurate measured values. Refined line-shape parameters such as the speed dependence of the broadening and the Dicke narrowing coefficient have a small impact on the line profile so that differences of several percent are completely negligible contrary to the linebroadening coefficients for which these differences can lead to large change on the calculated line shape.…”

“…Thanks to the increasing computer power and 3 in conjunction with the availability of more accurate full-dimensional potential energy surfaces, this method will be probably applied to more complex molecular systems in the future. On the other hand, line-shape parameters for various molecular systems such as CO2 perturbed by N2 and O2 [21,22], CO in various perturbers [23], O2 in air [24] were successfully predicted using requantized classical molecular dynamic simulations (rCMDS). The principle of these calculations is very simple: based on the classical time evolution of a system containing a large number of molecules, one can compute the time dependent auto-correlation function of the dipole moment, responsible for the transition.…”

“…The simulated spectrum can then be fit with phenomenological line-shape models yielding the corresponding line-shape parameters. This method was recently successfully used to predict line-shape parameters for N2- [21] and O2-broadened [22] CO2 lines, for instance.…”

Re-evaluation of ortho-para-dependence of self-pressure broadening in the $v_1+v_3$ band of acetylene

“…Understanding the variation of the shapes and widths of spectral lines as a function of pressure and composition is critical for remote sensing and other analytical applications and much effort has been expended in both measurement, calculation and modeling of observed line profiles. [1][2][3][4][5] Given our understanding of the phenomenon, it was therefore a surprise when, in 2016, Iwakuni et al 6 measured the well-studied v 1 + v 3 vibrational combination band of acetylene using a new instrument based on a dual frequency-comb [7][8][9][10][11][12] spectrometer and reported a large (≈ 10%) alternation in the measured self-pressure broadening coefficients with rotational quantum number. Transitions involving ortho-acetylene molecules apparently showed a greater self pressure broadening than para-ones.…”