Classical calculation of self-broadening in N2 Raman spectra

Abstract

This paper reports on a detailed study of self-broadening of Raman lines in gaseous N2 using classical trajectory method. Formulas of classical impact theory of spectral lines are used along with derived exact 3D Hamilton equations in body-fixed coordinates for N2–N2 binary collisions. Molecules are treated as rigid rotors, several available atom-atom + electrostatic intermolecular potentials are used. Q- and S-branch linewidths of pure N2 are computed for the rotational quantum numbers J = 0–30 at different temperatures ranging from 113 to 2400 K. The validity of mean thermal velocity approximation is tested. The results of calculations are compared with existing experimental data and with the predictions of semiclassical Robert–Bonamy method. The agreement between these classical results and experiment is found to be good, better than the results using the Robert–Bonamy method. Special consideration is given to temperature dependence of linewidths and its approximation by simple power law. The contributions of elastic and inelastic collisions to N2 self-broadening are analysed in the light of experimentally observed differences between Q- and S-branch linewidths. The simple and obvious classical method is found to be capable of accurate and rapid predictions of collisional line self-broadening of nitrogen molecules in binary collisions.