MEASUREMENTS OF ATOMIC SODIUM IN FLAMES BY ASYNCHRONOUS OPTICAL-SAMPLING - THEORY AND EXPERIMENT

Asynchronous optical sampling (ASOPS) is a pump-probe method for the measurement of species concentrations in turbulent high-pressure flames. We show that rapid measurement of species number densitY can be achieved in a highly quenched environment by maintaining a constant beat frequency between the mode-locking frequencies of the pump and the probe lasers. A model for the ASOPS method based on rate equation theory for three- and four-level atoms is presented. A number of improvements are made to the basic ASOPS instrument, which result in a greatly enhanced signal-to-noise ratio. Atomic sodium is aspirated into an atmospheric pressure C2H4/O2/N2 flame and detected with the ASOPS instrument. When excited-state lifetimes are fitted by using the ASOPS theory, a 3P1/2,3/2 --> 3S1/2 quenching-rate coefficient of 1.72 x 10(9) s-1 and a 3P3/2 --> 3P3/2 doublet-mixing rate coefficient of 3.66 x 10(9) s-1 are obtained, in excellent agreement with literature values. ASOPS signals obtained over a wide range of pump and probe beam powers validate the rate equation theory. Improvements are suggested to improve the signal-to-noise ratio, since the present results are limited to laminar flows.