Pressure, temperature, and density measurements using high-resolution N₂ cars

Mean pressure, temperature, and density measurements have been made in a gas cell and along the centerline of an underexpanded jet using highresolution N₂ CARS. This method takes advantage of the line-broadening effects and population shifts of the rotational structure in the nitrogen (u = 0 → 1) Q-branch which are pressure- and temperature-sensitive. Experimental CARS spectra were acquired with high spectral resolution, Δω = 0.16-0.18cm^-1. Theoretical spectra, created by a N2 spectral modeling program, were fit to the experimental spectra in a least-squares manner. Temperature and pressure were determined as adjustable parameters in the least-squares fit and density was then determined through an equation-of state. For gas cell pressures between 1 and 5 arm, the pressure values extracted from the CARS spectra exhibited an approximately 0.15 atm bias above the transducer pressures, probably because of Raman saturation effects. However, in the underexpanded jet measurements, our Stokes laser power was lower and agreement between measured and predicted pressures and temperatures along the jet centerline was excellent. The potential of performing pressure and temperature measurements on single laser shots was also examined.