A laser-absorption diagnostic for O2 concentration and temperature using a portable, tunable UV laser system

A portable ultraviolet (UV) laser absorption diagnostic was developed to measure temperature and O₂ concentration in high-temperature environments. The diagnostic uses two wavelengths (225.0150 nm/44,441.48 cm⁻¹; 225.0447 nm/44,435.62 cm⁻¹) to probe absorption features with components arising from two different lower vibrational levels of the Schumann–Runge system (\(B^3 \Sigma _u^-\leftarrow X^3 \Sigma _g^-\)). To ascertain the position of the features, absorption cross-section measurements were collected at a variety of wavelengths from 225.0000 to 225.0460 nm in a reflected shock tube. After identifying spectral peak locations, the temperature dependence of the absorption cross-section at each peak was measured from 1500 to 5000 K. Experimental measurements motivated changes to an existing spectroscopic model, enabling accurate temperature-dependent cross-section predictions at both wavelengths within experimental uncertainty. Diagnostic validation data shows accurate predictions of temperature and O₂ mole fraction across a wide range of conditions (T = 1600–4500 K; P= 0.15–0.90 atm; \(\chi _{O_2}\) = 2–100%). The average measurement error was 4% for both temperature and mole fraction. The diagnostic was also used to track O₂ dissociation as a function of time behind reflected shock waves and showed good agreement with an in-house coupled vibration-dissociation model.