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 (N₂), for example, which is a dominant species in air-fed combustion processes. Although hydrogen (H₂) 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 H₂ S-branch. We demonstrate H₂ thermometry using hybrid femtosecond/picosecond pure-rotational CARS, in which a broadband pump/Stokes pulse enables simultaneous excitation of the set of H₂ S-branch transitions populated at flame temperatures over the spectral region of 0-2200 cm^-1. We present a pure-rotational H₂ CARS spectral model for data fitting and compare extracted temperatures to those from simultaneously collected N₂ 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 H₂ and N₂ CARS extracted temperatures are, on average, within 2% of the set temperature. For flame measurements, the fitted H₂ and N₂ temperatures are, on average, within 5% of each other from 300 to 1600 K. Our results confirm the viability of pure-rotational H₂ CARS thermometry for probing combustion reactions.