Stimulated Raman spectroscopy of the Q branch of nitrogen at high pressure: collisional narrowing and shifting in the 150–6800 bar range at room temperature

Abstract

The Raman Q branch of N₂ has been recorded at room temperature in the pressure range 150–6800 bar, which corresponds to densities from 135 to 800 amagat. In this domain, the Raman Q branch profile is mainly determined by the well-known collisional narrowing. The experimental data have been obtained by means of a high resolution stimulated Raman spectrometer. The linewidth and line shift of the band have been accurately measured as functions of the density, and their density dependences have been fitted by polynomials. The minimum of the linewidth and the maximum of the red shift have been clearly observed at respectively 735·8 and 532·3 amagat. The lineshape was found to be Lorentzian above 288 amagat with a linewidth inversely proportional to the density up to 670 amagat. This led to the determination of the rotational relaxation constant nτ_E = 0·160 ns amagat. Above 670 amagat, a nonlinear increase in the linewidth due to the increasing influence of the vibrational contribution has been observed. Fitting laws usually employed to model the relaxation matrix and thus to account for the collisional narrowing have been tested against our experimental spectra. The Schweizer-Chandler model has been also applied to reproduce the density dependences of the Raman linewidth and line shift at high density.