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 × 109 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 × 1017 cm−3 is predicted for the Q 1 (9) line of A 2+ (v = 0)–X 2Π (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.

© 1995 Optical Society of America

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