Molecular dynamics simulations for CO2 spectra. IV. Collisional line-mixing in infrared and Raman bands

Abstract

Ab initio calculations of the shapes of pure CO2 infrared and Raman bands under (pressure) conditions for which line-mixing effects are important have been performed using requantized classical molecular dynamics simulations. This approach provides the autocorrelation functions of the dipole vector and isotropic polarizability whose Fourier-Laplace transforms yield the corresponding spectra. For that, the classical equations of dynamics are solved for each molecule among several millions treated as linear rigid rotors and interacting through an anisotropic intermolecular potential. Two of the approximations used in the previous studies have been corrected, allowing the consideration of line-mixing effects without use of any adjusted parameters. The comparisons between calculated and experimental spectra under various conditions of pressure and temperature demonstrate the quality of the theoretical model. This opens promising perspectives for first principle ab initio predictions of line-mixing effects in absorption and scattering spectra of various systems involving linear molecules.

Molecular dynamic simulations of N2-broadened methane line shapes and comparison with experiments

“…[15][16][17][18][19] For each considered transition and pressure, the absorption spectrum was fitted with the Voigt profile and a baseline. The Doppler half width Γ D is fixed to its theoretical value, calculated at the considered temperature while the line area S, the collisional half-width Γ L (P), the effective line-center position σ 0 (P), and the baseline parameters were adjusted together in the fit.…”

“…Each molecule m is then characterized by its center-ofmass position q m (t) and velocity˙ q m (t), by a unit vector u m (t) along a molecular axis and by its rotational angular velocitẏ u m (t). Note that unlike linear molecules, [15][16][17][18][19] methane is a spherical top with 4 hydrogen atoms evenly distributed on the surface of a sphere with a carbon atom located at its center. The molecular axis ( u m (t)) of methane is therefore chosen to lie along a given C-H bond.…”

“…(3), sign is the sign of the projection of the vector product of the rotational angular velocity˙ u m and the molecular axis u m (t) along the fixed axis x. 16,29 θ m (t) and thus the auto-correlation function of the dipole moment (carried by the molecular axis) are then calculated for each time step using this requantization procedure. The other parameters such as˙ u m and u m (t) are kept unchanged with respect to their classical values and follow the classical evolution of the system.…”

“…Specifically, the measured spectral shapes and their evolution with pressure are satisfactorily reproduced by the calculations. [14][15][16][17][18][19] Classical molecular dynamic simulations (CMDSs) were also successfully applied to non-linear molecules as for the case of pure H 2 O 20,21 and H 2 O diluted in N 2 . 22 In these situations, quaternion coordinates were used to treat the rotation of the molecules.…”

“…[14][15][16][17][18][19] From reliable intermolecular potentials, the evolution of a system involving a large number of molecules can be computed, including the time dependence of the position of the center of mass, of the translational and angular velocities, and of the orientation of each molecule. The auto-correlation function of the molecular dipole moment can be also computed.…”