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

Abstract

Spectral shapes of isolated lines of HCl perturbed by Ar are investigated for the first time using classical molecular dynamics simulations (CMDS). Using reliable intermolecular potentials taken from the literature, these CMDS provide the time evolution of the auto-correlation function of the dipole moment, whose Fourier-Laplace transform leads to the absorption spectrum. In order to test these calculations, room temperature spectra of various lines in the fundamental band of HCl diluted in Ar are measured, in a large pressure range, with a difference-frequency laser spectrometer. Comparisons between measured and calculated spectra show that the CMDS are able to predict the large Dicke narrowing effect on the shape of HCl lines and to satisfactorily reproduce the shapes of HCl spectra at different pressures and for various rotational quantum numbers.

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

“…Spectra of 8 isolated methane lines broadened by N 2 were recorded with a difference frequency laser spectrometer (DFLS) 19,30 at IEM-CSIC in Madrid, Spain. Briefly, tunable IR radiation is produced by difference frequency mixing of an Ar + laser and a tunable ring dye laser in LiNbO 3 crystals.…”

“…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.…”

“…The requantization scheme described in Ref. 19 has been applied here to the CH 4 angular momentum ω m (t). Specifically, one considers the angle θ m (t) between u m (0) and u m (t).…”

“…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.…”