Lineshape analysis of stimulated Raman spectra of the near-nozzle region of free jet expansions

An anomalous lineshape of stimulated Raman spectra obtained from the region very close to the nozzle of supersonic pulsed expansions of nitrogen is presented. High resolution Raman spectra of the Q branch of the fundamental vibration mode of N2 have been recorded from two different nitrogen expansions at T0 = 295 K and P0 = 1.5−3.5 bar, the lasers crossing the jet axis in the range z/D = 0.25 − 1.25, where D is the effective nozzle diameter. The combination of Doppler shifts and strong gradients of density and temperature in the near-nozzle region yield an inhomogeneous broadening and a double-peak structure of the recorded Raman line profiles. The comparation of the experimental results with the simulation of the Raman spectrum from this region provides valuable information about the near-nozzle flow field. The lineshape described here is different from another reported previously in the literature, which is based on a depletion of the density of free molecules on the axis due to condensation.