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Simultaneous Temperature and Pressure Measurements in Compressible Flow Using Nanosecond O2 Coherent Anti-Stokes Raman Spectroscopy

Simultaneous pure-rotational coherent anti-Stokes Raman spectroscopy (PRCARS) and vibrational O2 CARS spectroscopy (VCARS) were performed at elevated pressure and lowered temperature conditions in non-reacting compressible flow. We applied dual-pump CARS in a three-laser, three-color configuration to simultaneously acquire the PRCARS and VCARS spectra of O2. PRCARS spectra provide excellent sensitivity to temperature at relatively low temperatures. Pressure was extracted using the differential response of collisional effects in the PRCARS and the VCARS spectra. We used an under-expanded jet outside a choked converging nozzle as the compressible flow-field. We numerically analyze the pressure sensitivity of the combined CARS technique. Finally, we compare the collisional narrowing lineshape models of rotational diffusion narrowing and modified-exponential-gap model, for fitting the experimental spectrum. Graphical Abstract