High-resolution spectral analysis of ammonia near 6.2 μm using a cw EC-QCL coupled with cavity ring-down spectroscopy.

We report on the development of a mid-infrared cavity ring-down spectrometer (CRDS) coupled with a continuous wave (cw) external cavity quantum cascade laser (EC-QCL), operating between 6.0 μm and 6.3 μm, for high-resolution spectroscopic studies of ammonia (NH3) which served as a bench-mark molecule in this spectral region. We characterized the EC-QCL based CRDS system in detail and achieved a noise-equivalent absorption (NEA) coefficient of 2.11 × 10-9 cm-1 Hz-1/2 for a 100 Hz data acquisition rate. We thereafter exploited the system for high-resolution spectroscopic analysis of interference-free 10 transition lines of the ν4 fundamental vibrational band of NH3 centred at ∼6.2 μm. We probed the strongest interference-free absorption line RQ(4,3) of ν4, centred at 1613.370 cm-1 for highly-sensitive trace detection of NH3 and subsequently achieved a minimum detection sensitivity (1σ) of 2.78 × 109 molecules per cm3 which translated into the detection limit of 740 parts-per-trillion by volume (pptv/10-12) at a pressure of 115 Torr for an integration time of ∼167 seconds. To demonstrate the efficacy of the present system in real-life applications, we finally measured the mixing ratios of NH3 present in ambient air and human exhaled breath with high sensitivity and molecular specificity.