Knowledge Bank

OH in the burnt gases of a methane-air flames at atmospheric pressure is excited to individual $v\prime J\prime$ levels of the $A2\Sigma +$ state using a pulsed tunable laser. Measurements are made of the rotationally resolved fluorescent emission. Before radiating, the molecules undergo both rotational and vibrational collisional energy transfer, although they do not thermalize. The rotational population distributions must be described in terms of detailed state-to-state transfer rates, including a propensity for spin conservation (i.e.,$F1\to F1$ and $F2\to F2$ are favored over $F1\leftrightarrow F2$). The ration of quenching rate to rotational transfer rate increase with $N\prime$. Rotational transfer has significant effects on the determination of temperatures using excitation scans and finite bandpass detection. In general this leads to discrimination against high $J\prime$ lines, yielding temperatures several hundred degrees below the true values. This is apparent from both experimental measurements and from model calculations based upon The rotationally resolved fluorescence emission. Measurement of the upward $(v\prime =0\to 1)$ vibrational transfer, in conjunction with an estimation or measurement of the ratio of quenching rate to downward vibrational transfer rate and an assumption of detailed balancing, may be used to determine the temperature on a single pulse basis.