A N₂O laser absorption diagnostic near 4.6 μm for shock-tube chemical kinetics studies

Nitrous oxide is a NOx pollutant, a greenhouse gas, and an intermediate species involved in the combustion of nitrogenous fuels and propellants. To date, the high-temperature diagnostics employed in shock tubes to measure N2O time histories have utilized either IR emission or UV absorption. However, there appear to be no uses of IR absorption to measure N2O time histories at high temperatures. This paper presents the development of a N2O laser absorption diagnostic in the ν3 band of N2O at 4.56 μm. A set of three transitions near 2193 cm^-1, in the 00⁰1←00⁰0 and 01¹1←01¹0 bands, was selected. A scanned-wavelength setup was utilized, wherein the frequency of the quantum cascade laser output was rapidly scanned by modulating the injection current at 100 Hz (room-temperature measurements) or 2 kHz (hightemperature measurements). Both the static, room-temperature experiments and the hightemperature experiments behind the reflected shock wave demonstrated good agreement with an established spectroscopic database. This diagnostic shows promise for future measurements of N2O time histories behind reflected shock waves to aid in studies of NOx-related chemical kinetics.