ABSTRACT
Measurement of radical concentrations is important in understanding the chemical kinetics involved in combustion. Application of optical techniques allows for the nonintrusive determination of specific radical concentrations. One of the most challenging problems for investigators is to obtain flame data that are independent of the collisional environment. We seek to obviate this difficulty by the use of picosecond pump-probe absorption spectroscopy. A picosecond pump-probe absorption model is developed by rate-equation analysis. Implications are discussed for a laser-pulse width that is much smaller than the excited-state lifetime of the absorbing atom or molecule. The possibility of quantitative, quenching-independent concentration measurements is discussed, and detection limits for atomic sodium and the hydroxyl radical are estimated. For a three-level absorber-emitter, the model leads to a novel pump-probe strategy, called dual-beam asynchronous optical sampling, that can be used to obtain both the electronic quenching-rate coefficient and the doublet mixing-rate coefficient during a single measurement. We discuss the successful demonstration of the technique in a companion paper [Appl. Opt. 34, XXX (1995)].
ABSTRACT
We report the development of a pump-probe instrument that uses a high-repetition-rate (82-MHz) picosecond laser. To maximize laser power and to minimize jitter between the pump- and the probe-pulse trains, we choose the asynchronous optical sampling (ASOPS) configuration. Verification of the method is obtained through concentration measurements of atomic sodium in an atmospheric methane-air flame. For the first time to our knowledge, ASOPS measurements are made on a quantitative basis. This is accomplished by calibration of the sodium concentration with atomic absorption spectroscopy. ASOPS measurements are taken at a rate of 155.7 kHz with only 128 averages, resulting in a corresponding detection limit of 5 × 10(9) cm(-3). The quenching-rate coefficient is obtained in a single measurement with a variation of ASOPS, which we call dual-beam ASOPS. The value of this coefficient is in excellent agreement with literature values for the present flame conditions. Based on our quantitative results for detection of atomic sodium, a detection limit of 2 × 10(17) cm(-3) is predicted for the Q(1) (9) line of A (2)Σ(+) (v = 0)-X(2)II (v = 0) hydroxyl at 2000 K. Although this value is too large for practical flame studies, a number of improvements that should lower the ASOPS detection limit are suggested.
ABSTRACT
We demonstrate that picosecond mode-locked laser-based degenerate four-wave mixing can be detected with good signal-to-noise ratios in an optically thin flame and that detailed turbulence statistics can be acquired by use of this technique. A regeneratively mode-locked Ti:sapphire laser was tuned to the 4(2)S((1/2))-4(2)P degrees ((1/2)) transition in atomic potassium (which was doped into the flame) at 769.9 nm. Using the all-forward degenerate four-wave mixing geometry, we achieved signal-to-noise ratios of 70:1 without the use of a spatial filter. A sensitivity curve and a method for acquiring turbulence statistics are presented.
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.
ABSTRACT
Asynchronous optical sampling (ASOPS) is a pump-probe method that has strong potential for use in turbulent, high-pressure flames. We show that rapid measurement of species number density can be achieved by maintaining a constant beat frequency between the mode-locking frequencies of the pump and probe lasers. We also describe the instrumental timing parameters for ASOPS and consider the optimization of these parameters. Measurement of the nanosecond decay for electronically excited sodium in an atmospheric flame demonstrates the viability of the ASOPS technique in highly quenched flame environments.
