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) x 10(13) cm(-3) or 2.4 +/-1 ppm at 1900 K, with a flame-front width of 250 mum (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.
- ABSOLUTE CONCENTRATION MEASUREMENTS
- ROTATIONAL ENERGY-TRANSFER
- INDUCED PREDISSOCIATIVE FLUORESCENCE
- INDUCED SATURATED FLUORESCENCE
- CAVITY RINGDOWN SPECTROSCOPY
- DEPOSITING DC-ARCJET
- CONCENTRATION PROFILES