Angular momentum and rotational energy relaxation in N 2-N 2 collisions calculated from coherent and stimulated Raman spectroscopy data

Abstract

Rotational relaxation fitting and scaling laws, previously determined from coherent Raman spectroscopy of nitrogen, allow us to study the energy τ E and angular momentum τ J relaxation times. Consistent results with available experimental data are obtained and the high-temperature behaviour is discussed. To get a more detailed analysis of the collisional relaxation processes, new exact analytical expressions of the basic strong collision (SC) model for the Raman Q-branch are established for τ E and τ J . For the first time, quantum and quasiclassical descriptions of collisional narrowing of the Q-branch are thoroughly compared for the whole region of density variation manifest in the spectral transformation from the fully resolved isotropic Raman line up to the completely collapsed one. On this basis it is stated with confidence that: (1) scaling laws of quantum origin with adiabatic corrections are capable to excellently describe the resolved spectral structure and fail for the smoothed line under high densities; (2) contrary, the nonadiabatic quasiclassical model is in evident disagreement with state-to-state rates but suggests a qualitatively good prediction for the Q-branch isothermic narrowing. In our opinion these drastic features may indicate a change of the collisional regime and, in particular, an effective suppression of adiabatic channels of collisional relaxation in dense media.