Vacuum-ultraviolet frequency-modulation spectroscopy

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.

Ultra-sensitive spectroscopy of OH radical in high-temperature transient reactions

The hydroxyl (OH) radical is arguably the most important transient radical in high-temperature gas-phase combustion reactions, yet it is very difficult to measure because of its high reactivity and, thus, short lifetime and low concentration. This work reports the development of a novel method for ultra-sensitive, quantitative, and microsecond-resolved detection of OH based on UV frequency-modulation spectroscopy (FMS). To the best of the authors' knowledge, this is the first FMS demonstration in the near-UV spectral region for detection of short-lived radical species. Shot-noise-limited detection was achieved at an optical power of 25 mW. A proof-of-concept experiment in a tabletop H₂O/He microwave discharge cell has reached a 1σ minimum detectable absorbance (MDA) of less than 2×10^-4 over 1 MHz measurement bandwidth. High-temperature OH measurement was demonstrated in a 15 cm diameter shock tube, where a typical MDA of 3.0×10^-4 was achieved at 1330 K, 0.38 atm, and 1 MHz. These preliminary results have outperformed the previous best MDA by more than a factor of 3; further improvement by another order of magnitude is anticipated, following the strategies outlined at the end of this Letter. The current method paves the path to parts per billion (ppb) -level OH detection capability and offers prospects to significantly advance fundamental combustion research by enabling direct observation of OH formation and scavenging kinetics during key stages of fuel oxidation that were inaccessible with previous methods.