Broadband, background-free methane absorption in the mid-infrared

Rotationally resolved, broadband absorption spectra of the fundamental vibrational transition of the asymmetric C-H stretch mode of methane are measured under single-laser-shot conditions using time-resolved optically gated absorption (TOGA). The TOGA approach exploits the difference in timescales between a broadband, fs-duration excitation source and the ps-duration absorption features induced by molecular absorption to allow effective suppression of the broadband background spectrum, thereby allowing for sensitive detection of multitransition molecular spectra. This work extends the TOGA approach into the mid-infrared (mid-IR) spectral regime, allowing access to fundamental vibrational transitions while providing broadband access to multiple mid-IR transitions spanning similar to 150 cm(-1) (similar to 160 nm) near 3.3 mu m, thereby highlighting the robustness of this technique beyond previously demonstrated electronic spectroscopy. Measurements are conducted in a heated gas cell to determine the accuracy of the simultaneous temperature and species-concentration measurements afforded by this single-shot approach in a well-characterized environment. Application of this approach toward fuel-rich methane-nitrogen-oxygen flames is also demonstrated. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement