Using CMDS to study spectral shapes of methane broadened by nitrogen and comparison with experiment

Absorption spectra of methane transitions broadened by nitrogen have been calculated for the first time using a requantized classical molecular dynamic simulation [1]. For that, the time evolution of the auto-correlation function of the dipole moment vector, assumed along a C-H axis, was computed using accurate site-site intermolecular potentials for a CH4-N2 system [2-4]. A requantization procedure [5] was applied to the classical rotation and spectra were then derived as the Fourier-Laplace transform of the auto-correlation function. These computed spectra were compared with experimental ones recorded with a tunable diode laser spectrometer [6] and a difference-frequency laser one [7]. Specifically, nine isolated methane lines broadened by nitrogen, belonging to various vibrational bands and having rotational quantum numbers J from 0 to 9, were measured at room temperature and at several pressures from 20 to 945 mbar. Comparisons between measured and calculated spectra were made through their fits using the Voigt profile. The results show that ab initio calculated spectra reproduce with very high fidelity non-Voigt effects on the measurements and that classical molecular dynamic simulations can be used to predict spectral shapes of isolated lines of methane perturbed by nitrogen [8]