Effects of O2-CO2 polarization beating on femtosecond coherent anti-Stokes Raman scattering (fs-CARS) spectroscopy of O2

Abstract

Femtosecond coherent anti-Stokes Raman scattering (fs-CARS) spectroscopy has recently emerged as a promising laser-based temperature-measurement technique in flames. In fs-CARS, the broad spectral bandwidths of the pump and Stokes lasers permit the coupling of each ro-vibrational Raman transition via a large number of pump-Stokes photon pairs, creating a strong Raman coherence. However, the broad-bandwidth fs pulses also excite other molecular transitions that are in resonance. The polarization beating between these closely spaced Raman transitions can affect the coherence dephasing rate of the target molecule, making it difficult to extract accurate medium temperature. In a previous study our group investigated N2/CO polarization beating in N2 fs-CARS; in the present work we study O2/CO2 polarization beating in O2 fs-CARS. O2 fs-CARS can be particularly important for thermometry in non-air-breathing combustion in the absence of N2. The effects of O2/CO2 polarization beating are investigated in the temperature range 300-900 K at atmospheric pressure and also at 300 K for pressures up to 10 bar. Unlike in the N2/CO system, it was observed in the O2/CO2 system that the presence of CO2 can significantly alter the time evolution of the Raman coherence and, hence, affect the measured temperature. © 2010 The Author(s).