Pure rotational Coherent anti-Stokes Raman spectroscopy of water vapor and its relevance for combustion diagnostics

In this work, we report for the first time the rotational coherent anti-Stokes Raman spectroscopy (CARS) spectrum of water vapor, recorded through experiments at 400K and 670K for Raman shifts up to 600cm(-1). Using the standard dual-broadband pure rotational CARS approach, the resonant structure was overwhelmed by a strong non-resonant background. By employing a polarization technique, this background could be suppressed and the resonant structure discerned. The spectral line structure matched spectra from spontaneous rotational Raman scattering found in the literature. The CARS signal intensity at the highest peak from water vapor at 400K was more than five orders of magnitude weaker than for N-2 because of a low Raman cross section and a larger number of transitions. Due to the weak resonant signal, water vapor is an unsuitable candidate for CARS thermometry and relative concentration measurements in combustion. Still, rotational CARS spectra from product gases in combustion will be affected by the presence of water, which is briefly discussed. Copyright (c) 2013 John Wiley & Sons, Ltd.