Stark Broadening and Stimulated Raman Pumping in High-Resolution N Coherent Anti-Stokes Raman Scattering Spectra

Abstract

In previous studies, the high-resolution N 2 coherent anti-Stokes Raman scattering (CARS) technique has been used to acquire pressure, temperature, and density measurements in high-speed supersonic e ows. In these lowdensity e ows, a tradeoffexistsbetween elevating theCARS signal strength with increasing pump- and Stokes-laser intensities and introducing Stark broadening and stimulated Raman pumping effects into the high-resolution N 2 CARS spectra. To explore these laser-induced perturbations, theCARS 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 (D ! =0:10 cm i 1 ) broadband CARS spectra are explored. For pump-laser intensities greater than 185 GW/cm 2 , the least-squares e ts of the experimental spectra with theoretical spectra provide pressures and temperatures that diverge from conditions measuredwithin thecell usingconventional transducersforpressuresandtemperaturesaround0.2 atm and298K, respectively. An analytical model based on rigid-rotator harmonic-oscillator theory is used to describe how the increased optical e elds 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 pumpingeffects,areacquiredwithhighresolution.Least-squarese tsoftheseexperimentalspectrawiththeoretical spectra show that stimulated Raman pumping signie cantly 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.