Laser-induced perturbation effects in high-resolution N₂ CARS spectra

The high-resolution N₂ 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 370 GW/cm², the least-squares fits of the experimental spectra with theoretical spectra provide pressures and temperatures which diverge from conditions measured within the cell using conventional transducers. An analytical model based upon rigidrotator 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, inhomogeneously broadening and redistributing the population of the rotational transitions in the Q-branch manifold. Finally, by increasing the pump- and Stokes-laser intensities concomitantly, 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 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.