Ground state saturated population distribution of OH in an acetylene-air flame measured by two optical double resonance pump-probe approaches

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Abstract
Two optical double-resonance pump–probe techniques were used to determine the ground-state rotational population distributions of OH in an acetylene–air flame when a saturating laser beam is tuned to the Q14 transition of the (0, 0) Σ–Π band. The saturated absorption technique is based on the detection of absorption by a probe laser under conditions of saturation with a pump laser and no saturation. In the fluorescence technique, a probe laser is scanned through the (1, 0) band, while a saturating pump laser, tuned to the (0, 0) band, is on or off. We found that ∼15% of the total population of the ground state was transferred to the excited state. Perturbation of the rotational population distribution was greater for rotational levels close to the directly excited laser-coupled level. The rotational energy transfer rate in the ground state was somewhat greater than in the excited state. The assumption of the balanced cross-rate model was verified as a means of determining the absolute OH number density with adequate accuracy.