Pure-rotational H2 thermometry by ultrabroadband coherent anti-stokes Raman spectroscopy

Coherent anti-Stokes Raman spectroscopy (CARS) is a sensitive technique for probing highly luminous flames in combustion applications to determine temperatures and species concentrations. CARS thermometry has been demonstrated for the vibrational Q-branch and pure-rotational S-branch of several small molecules. Practical advantages of pure-rotational CARS, such as multi-species detection, reduction of coherent line mixing and collisional narrowing even at high pressures, and the potential for more precise thermometry, have motivated experimental and theoretical advances in S-branch CARS of nitrogen (N-2), for example, which is a dominant species in air-fed combustion processes. Although hydrogen (H-2) is of interest given its prevalence as a reactant and product in many gas-phase reactions, laser bandwidth limitations have precluded the extension of CARS thermometry to the H-2 S-branch. We demonstrate H-2 thermometry using hybrid femtosecond/picosecond pure-rotational CARS, in which a broadband pump/Stokes pulse enables simultaneous excitation of the set of H-2 S-branch transitions populated at flame temperatures over the spectral region of 0-2200 cm(-1). We present a pure-rotational H-2 CARS spectral model for data fitting and compare extracted temperatures to those from simultaneously collected N-2 spectra in two systems of study: a heated flow and a diffusion flame on a Wolfhard-Parker slot burner. From 300 to 650 K in the heated flow, the H-2 and N-2 CARS extracted temperatures are, on average, within 2% of the set temperature. For flame measurements, the fitted H-2 and N-2 temperatures are, on average, within 5% of each other from 300 to 1600 K. Our results confirm the viability of pure-rotational H-2 CARS thermometry for probing combustion reactions. Published by AIP Publishing.