Time-Resolved, NIR Two-Tone Frequency Modulation Spectroscopy for Quantitative Measurement of HO2 Radical in Fuel Oxidation Reactions
Time-resolved two-tone frequency modulation (TTFM) absorption spectroscopy has been used to measure, in situ and quantitatively, hydroperoxy (HO2) radical in fuel oxidation reactions at the first overtone transitions (2v1) of HO2 near 1509nm. Typical HO2 detection limit is on the order of 1011 molecule cm-3, which corresponds to a relative absorption of 10-5. TTFM method successfully removes low frequency thermal lensing noise in measured HO2 kinetic time traces, which is a general noise source in fuel oxidation absorption experiments. Compared with previous works, we have upgraded the TTFM experiment with a NIR distributed feedback (DFB) diode laser, a fiber-coupled broadband phase modulator, and a two-channel wave generator, which have improve the performance of our experiment substantially.