Single-Tone Mid-Infrared Frequency Modulation Spectroscopy for Gas-Phase Reaction Kinetics

A new single-tone mid-infrared frequency modulation (ST-MIR-FM) spectrometer has been developed and characterized. For the first time, ST-MIR-FM detection was realized by the use of an external electro-optic modulator and by all-digital demodulation. The ST-MIR-FM system reaches a minimum detectable absorption of 2.9×10−4 at 1 µs time resolution. A notable sensitivity improvement compared to conventional absorption spectroscopy with 2×10−3 was achieved.

UV photolysis of oxalyl chloride, (ClCO)2, was investigated to determine the quantum yield of direct Cl2 formation channel using time-of-flight mass spectrometry. Complementary ST-MIR-FM detection of HCl in (ClCO)2/C2H6/Ar mixtures served to trace the decomposition of the ClCO radical. Kinetic analysis of the Cl2 profiles provided quantum yields for direct Cl2 formation of up to 10%.

HCN could be detected with ST-MIR-FMS behind shock waves by targeting its P(26) line in the ν1 band. FM spectra of HCN were reconstructed from individual shock tube experiments around the absorption line. Lineshape analysis provided the pressure-broadening parameters of the P(26) line. For 1700 K and 1.2 bar, an HCN detection limit of 22 ppm at 1 µs time resolution was achieved.

Two combustion-related reactions of HCN were investigated. Firstly, the total rate constant of HCN+O was measured in the temperature range 1400 K - 2000 K, completing the available literature rate constants. Secondly, the channel branching of the prompt-NO switch reaction NCN+H was investigated. By simultaneous detection of NCN and HCN via UV laser absorption and ST-MIR-FMS, the branching fraction of the HCN-forming channel has been determined, yielding 0.22 to 0.63 within 1200 K - 2000 K. These experiments represent the first direct branching fraction measurements for NCN+H.

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