Hydrogen coherent antistokes Raman spectra from CH4–N2O and nitramine composite flames

Abstract

The H₂ spectra from the CH₄–N₂O and nitramine composite flame allow use of each of the concurrent capabilities of CARS: (i) rovibrational state resolution better than 1 cm^–1, (ii) time resolution better than 10 ns and (iii) spatial resolution better than 100 µm. The rovibrational state resolution results in direct observation of previously unobserved higher J transitions in the Q branch ν″= 0 and ν″= 1 and S branch ν″= 0 transitions. These transitions are in accord with transitions calculated from ab initio results and with transitions indirectly obtained from electronic emission spectra. The ab initio results give a compact set of constants that give results within the experimental accuracy of the CARS experimental results. The experimental transition energies have been used to construct Boltzmann plots that allow discrimination among the various spectra used to determine temperature. CARS is thus seen to provide the spectral resolution and intensity to allow direct observation of previously unobserved hydrogen higher J and ν rotational transitions. These transitions have been used to obtain temperature profiles in the CH₄–N₂O and nitramine composite flames which requires both the high spatial and temporal resolution of CARS. These temperature profiles have provided additional confirmation of previous kinetic mechanisms used to explain observations in the nitramine composite flame. These concurrent capabilities of CARS used here demonstrate the high potential of CARS for use as the probe beam in kinetic flash photolysis and direct in situ measurements of combustion flames.