Quantitative laser-induced fluorescence of CH in atmospheric pressure flames.

Absolute number densities of the CH radical were determined in a partially premixed methane/air flame (equivalence ratio was 1.36) at atmospheric pressure by exciting a predissociating level via the CH B-X(1,0) transition using a quasilinear laser-induced fluorescence scheme. The peak number density was (1.0±0.4) × 10[sup 13] cm[sup -3] or 2.4 ± 1 ppm at 1900 K, with a flame-front width of 250 µm (FWHM). Rotational energy transfer must be considered for correct laser-induced fluorescence signal interpretation. Competition between optical pumping and rotational relaxation in both excited and ground states produces a signal that varies almost linearly with laser pulse energy even for large pumping rates. For these conditions, the population of the initial ground-state rotational level is depleted by optical pumping, and rotational energy transfer collisions rapidly repopulate the level during the laser pulse. Deviations from linear behavior are less than 20%. The effects of spatial resolution and polarization of the fluorescence on the absolute measurements are also discussed. [ABSTRACT FROM AUTHOR]