LASER-INDUCED PREDISSOCIATIVE FLUORESCENCE - DYNAMICS AND POLARIZATION AND THE EFFECT OF LOWER-STATE ROTATIONAL ENERGY-TRANSFER ON QUANTITATIVE DIAGNOSTICS

Laser-induced predissociative fluorescence is often used for diagnosti cs because its short-lived upper states are minimally disturbed by col lisions. We discuss the effects of lower-state collisions with paramet ers relevant to our atmospheric H-2-O-2 flame. A pulse of tunable KrF excimer-laser light induces the A <-- X, Q(1)(11), 3 <-- 0 transition in OH. We measure the intensity and the polarization of the resulting A --> X, Q(1)(11), 3 --> 2 fluorescence as a function of laser brightn ess. A simple model that uses no adjustable parameters produces a reas onable fit to the data. It predicts that, even at very modest laser en ergies, the fluorescence intensity is almost directly proportional to the rate constant for rotational energy transfer (RET) within the lowe r vibrational state. That rate constant can be a strong function of lo cal conditions. Furthermore, under typical operating conditions the ex cimer will pump an amount of OH out of the lower state that is many ti mes as large as that originally present. This occurs because RET withi n the X-state continuously replenishes the lower state during the lase r pulse. Even when this occurs, the signal may still vary linearly wit h laser intensity, and the polarization may be nearly that expected fo r weak pumping. At the higher laser intensities, a significant fractio n of the measured OH arises from two-photon photodissociation of the w ater from the flame reaction. (C) 1996 Optical Society of America