Water vapor in hydrogen flames measured by time-resolved collisional dephasing of the pure-rotational N₂ CARS signal

We present a novel diagnostic technique to probe water vapor (H₂O) concentration in hydrogen (H₂) combustion environments via the time-resolved measurement of the collisional dephasing of the pure-rotational coherent anti-Stokes Raman scattering (CARS) signal of nitrogen (N₂). The rotational Raman coherence of the N₂ molecules, induced by the interaction with the pump and Stokes laser fields, dephases on a timescale of hundreds of picoseconds (ps), mostly due to inelastic collisions with other molecules in atmospheric flames. In the spatial region of H₂ flames where H₂O is present in appreciable amount, it introduces a faster dephasing of the N₂ coherence than the other major combustion species do: we use time-resolved femtosecond/picosecond (fs/ps) CARS to deduce the H₂O mole fraction from the dephasing effect of its inelastic collisions with N₂. The proof-of-principle is demonstrated in a laminar H₂/air diffusion flame, performing sequential measurements of the collisional dephasing of the N₂ CARS signal up to 360 ps. We measure the temperature and the relative O₂/N₂ and H₂/N₂ concentrations at a short probe delay, and input the results in the time-domain model to extract the H₂O mole fraction from the signal decay, thus measuring the whole scalar flow fields across the flame front. We furthermore present single-shot simultaneous thermometry and absolute concentration measurements in the turbulent TU Darmstadt/DLR Stuttgart canonical 'H3 flame' performed by dual-probe CARS measurements obtained with a polarization separation approach. This allows us to probe the molecular coherence simultaneously at -20 and -250 ps on the basis of a single-laser-shot, and record the resulting signals in two distinct detection channels of our unique polarization-sensitive coherent imaging spectrometer. The proposed technique allows for measuring the absolute concentrations of all the major species of H₂ flames, thus providing a full characterization of the flow composition, as well as of the temperature field.