Discharge modulation technique in a supersonic jet for infrared diode laser absorption spectroscopy

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Ohio State University

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A new discharge modulation technique for the detection of transieut molecules in the supersonic jet has been devised for the infrared diode laser spectroscopy. Comer and $Faster^{1}$ have reported a discharge modulation method combined with corona discharge. Carbon disulfide diluted to 3% with A atm Ar was injected in an absorption cell through a discharge nozzle with a repetition rate of 6 Hz. A cathode made of 4 mm id tantalum tube and isolated with a retion plate was attached to a commercial solenoid valve with 2 mm circular orifice. The cathode was surrounded by an anode made of a 50 mm id stainless steel tube. Between the electrodes. 20 kHz ac high voltage pulse was applied for the duration of 1 ms, synchronized with the pulsed jet, to cause stable ac discharge of 100 mA, typically. Focused diode laser bearu in a Perry-type multireflection path crossed the molecular jet, beam 16 time just below ($2\sim 6$ mm). As the first rest of the nozzle, we observed the R (3) line in the fundamental band of CS radical in the time as well as frequency domain. The change in the density of CS was clearly observed with 50 $\mu$m/s repetition corresponding to the 20 kHz rectified discharge current, although CS is almost stable. The CS radical was generated at the throat of the cathode and expanded with the speed of $\sim500 m/s$ outside of the discharge nozzle to be cooled down to a rotational temperature of a few tens of K. Infrared signal was demodulated by a phase sensitive detector with a time constant of 1 ms and averaged by a boxear integrator. The present discharge modulation resulted in an improvement of the S/N ratio by $3 \sim 4$ times.


1. K. R. Comer and S. C. Foster, Chem. Phys. Lett., 202, 216 (1993).
Author Institution: Department of Chemistry, Faculty of Science, Kynslm University