Stark broadening and stimulated Raman pumping in high-resolution N₂ coherent anti-stokes Raman scattering spectra

In previous studies, the high-resolution N₂ coherent anti-Stokes Raman scattering (CARS) technique has been used to acquire pressure, temperature, and density measurements in high-speed supersonic flows. In these low-density flows, a tradeoff exists between elevating the CARS signal strength with increasing pump- and Stokes-laser intensities and introducing Stark broadening and stimulated Raman pumping effects into the high-resolution N₂ CARS spectra. To explore these laser-induced perturbations, the CARS technique is used to acquire v = 0 → 1 and v = 1 → 2 CARS spectra over a range of pressures in an optically accessible gas cell. By controlling the intensity of the pump- and Stokes-laser beams, Stark broadening effects in the high-resolution (Δω = 0.10 cm⁻¹) broadband CARS spectra are explored. For pump-laser intensities greater than 185 GW/cm², the least-squares fits of the experimental spectra with theoretical spectra provide pressures and temperatures that diverge from conditions measured within the cell using conventional transducers for pressures and temperatures around 0.2 atm and 298 K, respectively. An analytical model based on rigid-rotator harmonic-oscillator theory is used to describe how the increased optical fields of the pump and Stokes lasers stretch the molecular bond between the nitrogen nuclei, broadening and shifting the rotational transitions in the Q-branch manifold. Finally, by increasing the pump- and Stokes-laser intensities simultaneously, ambient v = 1 → 2 N₂ CARS spectra, resulting from stimulated Raman pumping effects, are acquired with high resolution. Least-squares fits of these experimental spectra with theoretical spectra show that stimulated Raman pumping significantly increases the vibrational temperature extracted from the experimental spectra. The relative intensities of the rotational transitions in the v = 0 → 1 manifold, however, are not affected by the stimulated Raman pumping process.