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

Abstract

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 N 2 have been recorded from two different nitrogen expansions at T 0 = 295 K and P 0 = 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 comparison 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.

Femtosecond Rotational Raman Coherence Spectroscopy of Cyclohexane in a Pulsed Supersonic Jet

“…This is 0.42 MHz lower than the B 0 determined here and is outside the combined error bars. On the basis of a structural fit to the data of ref , Białkowska-Jaworska et al previously predicted B 0 = 4305.84(15) MHz for cyclohexane . This value is in excellent agreement with the supersonic-jet B 0 .…”

“…Raman spectroscopic measurements in molecular beams have been undertaken for a number of molecules ,,− including cyclohexane . The main benefit of using a molecular beam instead of a gas cell setup is the reduction of many perturbing factors .…”

Recent advances in linear and nonlinear Raman spectroscopy. Part IV

“…The signals are background (noise) free, unlike in CARS, and provide natural rejection of fluorescence, which is a major problem in Raman spectroscopy. Anomalous SRS line shapes in regions very close to the nozzle of supersonic pulsed expansions of N 2 are presented by Ramos et al [137] The comparison of the experimental results with the simulation of the SRS spectrum from this spatial region provides valuable information about the near-nozzle flow field. The high-resolution SRS of the band located at 1664.16 cm −1 in C 2 H 4 was reanalyzed by the tensorial formalism developed in Dijon for X 2 Y 4 asymmetric-top molecules.…”

Line shape in a free-jet hypersonic expansion investigated by cavity ring-down spectroscopy and computational fluid dynamics

“…Nevertheless, in their collaborative efforts, Campbell and Lovas finally concluded that density and temperature gradients along the laser beam are responsible for double peaked lines [10]. Ramos et al disregarded condensation of nitrogen and attributed the observed double peak structure of lines to inhomogeneous line broadening due to strong temperature and density gradients within the silent zone, along the line of sight [11].…”