Measurements of atomic sodium in flames by asynchronous optical sampling: theory and experiment

Abstract

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 C(2)H(4)/O(2)/N(2) flame and detected with the ASOPS instrument. When excited-state lifetimes are fitted by using the ASOPS theory, a 3P((1/2),3/2) ? 3S((1/2)) quenching-rate coefficient of 1.72 x 10(9) s(-1) and a 3P(3/2) ? 3P((1/2)) doublet-mixing rate coefficient of 3.66 x 109 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.