Spatially Resolved Absorption Spectroscopy of NO in Shock Tube Experiments Using an Ultraviolet Tunable Modeless Laser

Shock tube experiments are conducted in the Oxford T6 Stalker Tunnel utilising a 226 nm bespoke modeless laser for nitric oxide absorption spectroscopy. The laser is expanded into a laser sheet to spatially resolve absorbed light behind the shock wave. Each laser pulse comprises two discrete wavelengths, which can be independently tuned within the 224--227 nm range. Nitric oxide (NO) absorption lines are targeted to compare experimentally measured absorbance to NESS--NEQAIR simulations at the nominal experimental conditions and infer number densities through a simple least-squares minimisation. The T6 Stalker facility is used to drive shocks in the 3.3--6.5 km/s velocity range, with post-shock pressures varying between 35.8 and 150.7 kPa. The collection optics used for imaging were developed and optimised for VUV emission spectroscopy and designed as a telecentric system. Their performance and resolution are briefly presented in this paper.