Comparison and analysis of rotationally inelastic collision models describing the Q-branch collapse at high density

Abstract

The fitting laws usually employed to model the relaxation matrix and, therefore, to account for the collisional narrowing of the Q-branch have been tested against the density dependence of the isotropic Raman band width (Λ) for nitrogen. The behavior of Λ has been theoretically studied in a wide density range: from 10 to 1072 amagat at 295 K in gas and liquid phases. The independent binary collision (IBC) approach has been applied to reproduce the density dependence of Λ in this domain. Quite surprisingly, the IBC expression reproduces the density variation of Λ rather well, despite the limited validity of such an approach. Rotational energy relaxation cross sections (σ′ E and σ E) are also employed for comparison. Relaxation models that do not provide the experimental value of σ E cannot be used to describe the Q-branch collapse in the limit of extreme narrowing. We find that at the stage of narrowing the reconstructed dephasing rate is essentially dependent on the accepted model of rotational relaxation.