In this paper, photothermal deflection spectroscopy (PDS) is applied to measure ethylene in nitrogen in a flowing stream with velocities high enough (>100 cm/sec) that forced convection dominates over thermal conduction in the heat flow. A crossed laser beam geometry (Fig. 1) is used where the pump laser is a CO2 laser and the probe is a He–Ne laser. The probe laser is deflected as a result of an index of refraction gradient formed by the absorption of the CO2 laser by ethylene. This deflection is measured by a position sensitive detector. The PDS signal as a function of ethylene concentration is shown in Fig. 2 where a linear dependence was found. It was also linear with pump laser power and varied inversely with gas velocity. The PDS signal was also measured as a function of the relative position of the laser beam waists with respect to each other and with respect to the position of the tube from which the gas flowed. In addition, the energy equation for this system was solved analytically for the case of a steady-state temperature and numerically for the more general time-dependent case. The resulting calculations of signal as a function of time compare very well with experiment. This technique has the advantages of high sensitivity (1 ppm in the present conditions) and good spatial resolution, and it is nonintrusive. (12 min)

© 1984 Optical Society of America

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