Detection of C2H2 and HCl using mid-infrared degenerate four-wave mixing with stable beam alignment: towards practical in situ sensing of trace molecular species

A stable and convenient optical system to realize the forward phase-matching geometry for degenerate four-wave mixing (DFWM) is demonstrated in the mid-infrared spectral region by measuring DFWM signals generated in acetylene (C2H2) and hydrogen chloride (HCl) molecules by probing the fundamental ro-vibrational transitions. IR laser pulses tunable from 2900 cm(-1) to 3350 cm(-1) with a 0.025 cm(-1) linewidth were obtained using a laser system composed of an injection seeded Nd:YAG laser, a dye laser, and a frequency mixing unit. At room temperature and atmospheric pressure, a detection limit of 35 ppm (similar to 9.5x10(14) molecules/cm(3)) for C2H2 was achieved in a gas flow of a C2H2/N-2 mixture by scanning the P(11) line of the (010(11)(0))-(0000(0)0(0)) band. The detection limit of the HCl molecule was measured to be 25 ppm (similar to 6.8x10(14) molecules/cm(3)) in the same environment by probing the R(4) line. The dependences of signal intensities on molecular concentrations and laser pulse energies were demonstrated using C2H2 as the target species. The variations of the signal line shapes with changes in the buffer gas pressures and laser intensities were recorded and analyzed. The experimental setup demonstrated in this work facilitates the practical implementation of in situ, sensitive molecular species sensing with species-specific, spatial and temporal resolution in the spectral region of 2.7-3.3 mu m (3000-3700 in cm(-1)), where various molecular species important in combustion have absorption bands.