Knowledge Bank

Previously, we reported a pulsed discharge modulation technique combined with infrared diode laser absorption spectroscopy in pulsed supersonic $jet1.$ Using this technique, infrared absorption spectrum of the $\nu 2$ fundamental band of the $CH3$ radical was measured. Acetone diluted to 5% with 3 atm Ar was injected in an absorption Cell through a discharge nozzle with a repetition rate of 30 Hz. Between the electrodes, 5 kHz ac high voltage pulse was applied for the duration of 1 ms, synchronized with the pulsed jet, to cause stable ac discharge of $\sim 50$ mA. Focused diode laser beam along the Perry-type multireflection path crossed the molecular jet beam 16 times just below $(2\sim 6mm)$. Infrared signal was demodulated by a phase sensitive detector with 1 ms time constant and averaged by a boxear integrator. We measured the $Q2(2)$ and $Q3(3)$ lines in the $\nu 2$ fundamental band of the $CH3$ radical. Rotational temperature was calculated to be 80 K from the intensity ratio of the $Q2(2)$ and $Q3(3)$ lines. Time profile of the transient infrared absorption of the CH3 radical was measured. The time profile indicated that $CH3$ was generated at the throat of the cathode and expanded into the vacuum chamber and cooled to 80 K.