A general model for the spectral calculation of OH radiation in the ultraviolet

A spectroscopic/collisional-radiative model is presented that permits the calculation of OH (A²Σ⁺ - X²Π) spectra in the ultraviolet for a wide range of gas dynamics conditions, including those observed in hypersonic flows. This requires a rotational-vibrational-electronic excitation model sufficiently general to account for the diversity of collision rates that encompasses conditions ranging from equilibrium to collisionless regimes. To this end, the proposed excitation model considers several state-specific processes, including quenching, predissociation, radiation, and vibrational-rotational exchange. Although the structure of the OH (A → X) spectra is complex, accurate line positions and transition probabilities are available and have been used. This paper discusses the full excitation and spectral model and provides examples that demonstrate its generality. Detailed comparisons are presented with experimental spectra measured under very different conditions. The first spectrum was measured at atmospheric pressure in a plasma generated by a 50 kW radio-frequency plasma torch. The second spectrum was measured under rarefied conditions in the bow shock of a reentering rocket.