Femtosecond, fully resonant electronically enhanced CARS (FREE-CARS) for simultaneous single-shot thermometry and detection of minor combustion species

Femtosecond time-resolved, fully resonant electronically enhanced coherent anti-Stokes Raman scattering (FREECARS) spectroscopy, incorporating a two-color ultraviolet excitation scheme, is used to demonstrate chemically selective and sensitive detection of gas-phase species, including nitric oxide (NO) and the hydroxyl (OH) radical. The observed time-dependent, spectrally resolved CARS signal contains rich structure that depends both on the rovibronic states accessed within the bandwidth of the initial (pump) excitation pulse and the Raman-active rovibrational levels within the vibrationally excited ground electronic state that are accessed following interaction with the second (Stokes) excitation pulse. By comparing experimental spectra to computational simulations, therefore, this approach also allows simultaneous determination of local temperature associated with the thermal distribution of initial states under singlelaser- shot conditions. For OH radical detected in a reacting flow, spectral resolution of the emitted FREE-CARS signal allows simultaneous single-shot detection of relative OH mole fraction and temperature in a laminar ethylene–air flame at 1-kHz repetition rates. By comparison to previously reported OH concentration and temperature measurements, we demonstrate excellent single-shot temperature accuracies (~2% deviation from adiabatic flame temperature) and precisions (~2% standard deviation), with simultaneous relative OH concentration measurements that demonstrate high detection sensitivity (100–300 ppm).