Cw cavity ring-down infrared absorption spectroscopy in pulsed supersonic jets: nitrous oxide and methane

We introduce cw cavity ring-down spectroscopy (cw-CRDS) in pulsed supersonic jet expansions employing a tuneable near-infrared cw diode laser and a solenoid slit nozzle. The cavity is mode-matched to the laser wavelength during the gas pulses to achieve highest resolution and lowest noise level. The new technique is characterised by observing very weak rovibrational lines of the ν1+3ν3 combination band of nitrous oxide near 7780 cm−1 and the ν2+2ν3 combination band of methane near 7510 cm−1. We demonstrate the increased spectral resolution due to the reduced Doppler width of rovibrational transitions and the spectral simplification afforded by the rotational cooling.