Reconstruction of the benzene absorption band at 14.8 μm in amplitude modulation exploiting a quartz tuning fork as optical detector

This work presents a single-pass optical sensor designed for the detection of a broadband absorption feature of benzene. It is based on light-induced thermoelastic absorption spectroscopy (LITES) employing a novel InAs/ AlSb-based quantum cascade laser source emitting at 14.85 mu m as light source, and a custom-designed quartz tuning fork (QTF) as infrared photodetector. The sensor system was operated in both amplitude (AM) and wavelength modulation (WM) mode to characterize the absorption band parameters and assess the sensor ultimate detection limit. The limitations of WM and double-frequency detection (2f-WM) approach for broadband absorption features are thoroughly analyzed and discussed. An absorption cross-section of (1.1 +/- 0.1) center dot 10 cm2 was measured in AM mode with a minimum detection limit of 1.6 part-per-million, while a minimum detection limit of 0.6 part-per-million was achieved using the 2f-WM mode, at 0.1 s of integration time, employing a 12 cm long single-pass cell.