High-sensitivity 308.6-nm laser absorption diagnostic optimized for OH measurement in shock tube combustion studies

We report the development of a high-sensitivity laser absorption diagnostic optimized for measurement of the hydroxyl radical (OH) at temperatures relevant to combustion studies, and demonstrated here in shock tube experiments. This diagnostic utilizes a narrow-linewidth CW UV laser that is tunable over the A(2)Sigma -X-2 Pi (0,0) band of the OH rovibronic transitions. First, we identified the strongest absorption transition of OH, over the current temperature range of interest, to be the Q(1)(5) transition near 308.61 nm. We then measured the OH absorption coefficients behind reflected shock waves over temperatures of 1656-2993 K and pressures of 0.88-4.09 atm, and determined the pressure-broadening and pressure-shifting coefficients in argon bath gas. Compared to the previous diagnostic targeting the OH R-1(5) transition, the current diagnostic has approximately 2.2 times the sensitivity. Finally, we demonstrated the excellent sensitivity of the current OH diagnostic in a set of highly-diluted C3H8 oxidation experiments in a shock tube, where a 1 - sigma detection limit of less than 0.15 ppm OH was successfully achieved.