Abinitiocalculations of the spectral shapes of CO2isolated lines including n

Abstract

We present a fully ab initio model and calculations of the spectral shapes of absorption lines in a pure molecular gas under conditions where the influences of collisions and of the Doppler effect are significant. Predictions of the time dependence of dipole autocorrelation functions (DACFs) are made for pure CO 2 at room temperature using requantized classical molecular dynamics simulations. These are carried, free of any adjusted parameter, on the basis of an accurate anisotropic intermolecular potential. The Fourier-Laplace transforms of these DACFs then yield calculated spectra which are analyzed, as some measured ones, through fits using Voigt line profiles. Comparisons between theory and various experiments not only show that the main line-shape parameters (Lorentz pressure-broadening coefficients) are accurately predicted, but that subtle observed non-Voigt features are also quantitatively reproduced by the model. These successes open renewed perspectives for the understanding of the mechanisms involved (translational-velocity and rotational-state changes and their dependences on the molecular speed) and the quantification of their respective contributions. The proposed model should also be of great help for the test of widely used empirical line-shape models and, if needed, the construction of more physically based ones.

Spectral shapes of Ar-broadened HCl lines in the fundamental band by classical molecular dynamics simulations and comparison with experiments

“…This requantization was applied only when the torque due to intermolecular interactions is below a properly chosen limit in order to let rotational-speed changes build up during collisions. 19,20 Thanks to the small values of the rotational constants of CO 2 and O 2 , the changes of ω i induced by this requantization do not influence the evolution of the system and its energy is thus conserved. For HCl which has a much larger rotational constant, a new requantization scheme has been used.…”

“…As detailed in Ref. 19 for instance, various collision-related effects affecting the shape of an isolated line are taken into account in the calculation. Comparison between CMDS-calculated spectra and measurements for both CO 2 and O 2 absorption lines at different pressures and in various spectral regions showed very good agreement.…”

“…Comparison between CMDS-calculated spectra and measurements for both CO 2 and O 2 absorption lines at different pressures and in various spectral regions showed very good agreement. [19][20][21][22] In the present work, CMDS are thus used for the calculation of the absorption line shapes of HCl diluted in Ar. For that, a new requantization procedure has been adopted in order to represent the rotational motion of HCl molecules.…”

“…In recent years, classical molecular dynamics simulations (CMDS) have been successfully used to predict the line shapes of linear molecules such as CO 2 or O 2 without use of any adjustable parameter. [19][20][21][22] From reliable intermolecular potentials, one can calculate the auto-correlation function of the dipole moment and then the absorption spectrum. Using a requantization procedure for the classical molecular rotation, the spectrum of a line can be calculated and then compared with the measured profile.…”

Velocity-changing collisions in pure H2 and H2-Ar mixture

“…calculations based on quantum mechanical scattering and solving transport/relaxation equation [19][20][21] or based on classical molecular dynamic simulations (CMDS) 22,23 can significantly reduce the number of approximations for spectra evaluation.…”