Sensitive Spectroscopy of Acetone Using a Widely Tunable External-Cavity Quantum Cascade Laser

We employed a single-mode, widely tunable (similar to 300 cm(-1)) external-cavity quantum cascade laser operating around 8 mu m for broadband direct absorption spectroscopy and wavelength modulation spectroscopy where a modulation frequency of 50 kHz was employed with high modulation amplitudes of up to 10 GHz. Using a compact multipass cell, we measured the entire molecular absorption band of acetone at similar to 7.4 mu m with a spectral resolution of similar to 1 cm(-1). In addition, to demonstrate the high modulation dynamic range of the laser, we performed direct absorption (DAS) and second harmonic wavelength modulation spectroscopy (WMS-2f) of the Q-branch peak of acetone molecular absorption band (HWHM similar to 10 GHz) near 1365 cm(-1). With WMS-2f, a minimum detection limit of 15 ppbv in less than 10 s is achieved, which yields a noise equivalent absorption sensitivity of 1.9 x 10(-8) cm(-1) Hz(-1/2).