Single-tone mid-infrared frequency modulation spectroscopy for sensitive detection of transient species

A single-tone mid-infrared frequency modulation (MIR-FM) spectrometer consisting of a cw-OPO-based laser system, a 500 MHz LiTaO ₃ electro-optical modulator (EOM), and a high-bandwidth GaAs mid-infrared detector has been developed. In order to assess the instrument's sensitivity and time resolution, FM spectra of selected CH ₄ transitions around 3070 cm ^-1 were measured and the reaction Cl + CH ₄ following the 193 nm excimer laser photolysis of oxalyl chloride was investigated by recording concentration-time profiles of HCl at 2925.90 cm ^-1 in a low-pressure slow-flow reactor. Furthermore, OH radicals were generated by UV photolysis of H ₂O ₂ and its transients were recorded at 3447.27 cm ^-1. The minimal detectable absorption of the spectrometer was determined to be A _min=4⋅10^-4 (Δ f _BW=1 MHz, ν~=3447 cm ^-1) by using the Allan approach. Mainly due to thermal noise contributions of the easy-to-saturate photodetector, the detection limit is about a factor of 4 above the shot-noise limit. To the best of our knowledge, this work reports the first implementation of a single-tone MIR-FM spectrometer based on an external EOM modulation scheme and its use for the detection of transient molecular species.