Fig. 1. Temperature dependence of Boltzmann fraction for different rotational levels in the state X²π (ν″ = 0) of OH.

Fig. 2. Typical laser spectrum and absorption spectra of line Q₁(8) of the OH A–X (1,0) band at (a) different pressures and 2000 K, (b) different temperatures and 10 bar. Panels (c) and (d) show the corresponding calculated overlaps assuming the laser frequency is tuned to the center of absorption spectra.

Fig. 3. (a) Calculated overlaps of Q₁(8) for the tuning and detuning cases at 2000 K and 10 bar, with the laser linewidth varying from 0 to 1 cm⁻¹. The detuning-induced overlap decreases are also calculated and shown in the figure. (b) The detuning-induced overlap decreases for different temperatures and pressures, where the laser linewidth is 0.1 cm⁻¹.

Fig. 4. Temporal profiles of the simulated fluorescence for CH₄/air flame: (a) at different pressures of 1 bar, 10 bar, 20 bar, 30 bar, and constant temperature of 1500 K; (b) at different temperatures of 1000 K, 1500 K, 2000 K, 2300 K, and constant pressure of 10 bar. The laser profile is also shown and marked with solid downward triangles in the figure. The amplitudes of the laser pulse are individually set according to the scales of y-axes in each sub-figures.

Fig. 5. (a) Relative fluorescence yields as a function of pressure at different temperatures. (b) Relative fluorescence yields as a function of temperature at different pressures. Note the logarithmic scale in sub-figure (b).

Fig. 6. OH fluorescence signal as a function of pressure for the Q₁(8) of A–X (1,0) at about 5 mm above the burner exit.