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 supersoni c jet expansions employing a tuneable near-infrared cw diode laser and a so lenoid slit nozzle. The cavity is mode-matched to the laser wavelength duri ng the gas pulses to achieve highest resolution and lowest noise level. The new technique is characterised by observing very weak rovibrational Lines of the nu(1) + 3 nu(3) combination band of nitrous oxide near 7780 cm(-1) a nd the nu(2) + 2 nu(3) combination band of methane near 7510 cm(-1). We dem onstrate the increased spectral resolution due to the reduced Doppler width of rovibrational transitions and the spectral simplification afforded by t he rotational cooling. (C) 1999 Elsevier Science B.V. All rights reserved.