A mid-IR laser diagnostic for HCN detection

Hydrogen cyanide (HCN) is a major source of prompt-NO_x formation especially in fuel-bound nitrogen systems. To date, there is still a significant disagreement between experimental data and theoretical predictions of the rate coefficients of combustion reactions involving HCN as a prompt-NO_x precursor. Accurate modeling of NO_x formation would greatly benefit from a diagnostic capable of performing high-fidelity measurements of HCN formation/consumption time-histories. In this study, a laser diagnostic is developed for sensitive and selective HCN sensing by probing its most intense absorption feature in the mid-infrared (MIR). The diagnostic is based on difference-frequency generation (DFG) between a CO₂ gas laser and an external-cavity quantum cascade laser in a nonlinear orientation-patterned gallium arsenide crystal which results in a DFG laser tunable over 11.56 − 15 µm. HCN measurements were carried out at the peak of the Q-branch of its strong ν₂ vibrational band near 14 µm. Pressure dependence of the absorption cross-section was investigated at room temperature over the pressure range of 0.07 − 1.07 bar. Temperature-dependent absorption cross-section measurements were conducted behind reflected shock waves over the temperature range of 850 − 3000 K. The diagnostic was demonstrated in reactive experiments in a shock tube where HCN mole fraction time-histories were measured during the thermal decomposition of isoxazole (C₃H₃NO) and the first-order rate coefficients of C₃H₃NO → HCN + CH₂CO reaction were determined.