Direct and 2f-wavelength modulation spectroscopy of NO and OCS using an astigmatic multipass cell coupled with a mid-IR 5.2 µm cw-QCL

Abstract We developed a mid-IR 2 f -wavelength modulation spectroscopy (WMS) detection system combining a continuous wave external-cavity quantum cascade laser (QCL) at 5.2 μ m and an astigmatic multipass cell. The high-sensitivity performance of the 2 f -WMS system was validated by simultaneously probing the strongest interference-free absorption lines of Λ-doublet components of R (6.5) rotational line in 2 Π 1/2 magnetic electronic sub-state of nitric oxide (NO) centred at 1900.0706 cm −1 and the R (18) rotational line of carbonyl sulphide (OCS) in the ( ν 1 + 2 ν 2 ) combination band centred at 1899.9756 cm −1 . We compared the results with the direct absorption spectroscopy and have shown that a detection limit of 300 ppb for NO and 3 ppm for OCS could easily be achieved by the 2 f -WMS method with an optical pathlength of 76 m. Hence, we believe that in the future the present QCL based 2 f -WMS detection method with high sensitivity and specificity could be deployed for real-time monitoring of NO and OCS in a harsh environment.