Rotational coherent anti-Stokes Raman spectroscopy (CARS) in nitrogen at high pressures (0.1-44 MPa): experimental and modelling results

Abstract

Pure rotational coherent anti-Stokes Raman scattering (CARS) has for the first time been investigated for pressures up to 44 MPa in nitrogen gas at room temperature. An atomic filter consisting of a heated cell with sodium vapour was successfully applied for suppression of stray light originating from the narrowband CARS pump laser beam. With increasing pressure the rotational CARS spectrum is smoothed gradually, and above around 10 MPa spectral lines are no longer resolvable. Experimental data were compared using Raman linewidths calculated with three different models: the energy corrected sudden scaling law (ECS), the modified exponential gap model (MEG), and a semi-classical ab initio model. All three models resulted in bad spectral fits when a linear scaling with pressure was employed. By using additional scale factors for the Raman linewidths the quality of the spectral fits and also the temperature accuracy were significantly improved. The resulting scale factors indicate a non-linear pressure dependence of the linewidths, and visualize a narrowing of the spectral envelope at the highest pressure, i.e. 44 MPa. The results indicate shortcomings in the present isolated line models and also emphasize the need for new experimental data on pure rotational Raman linewidths at high density. Copyright (C) 2000 John Wiley and Sons, Ltd. (Less)