Hook Spectroscopy as a Single-Shot OH Density and Temperature Diagnostic

Hook spectroscopy is being implemented to measure OH density and temperature in a premixed, CH4/air, atmospheric pressure flame on a single shot basis. The hook method makes use of the region of anomalous dispersion surrounding electronic transitions, typically in atomsl. In this region, the refractive index of the sample experiences dramatic shifts from its non-resonant values as shown in Figure 1. As seen in the figure, the solid curve has a minimum on the blue side of λ0, and a maximum to the red. The dashed curve represents the Sellmeier approximation to the refractivity which assumes that the linewidth is infinitely narrow. This approximation is normally quite valid in the wings of electronic transitions and is used to simplify the mathematics involved in determining number density from hook spectra.