GHz repetition rate mid-infrared frequency comb spectroscopy of fast chemical reactions

Abstract

Molecular diagnostics are a primary tool of modern chemistry, enabling researchers to map chemical reaction pathways and rates to better design and control chemical systems. Many chemical reactions are complex, involving multiple species and reaction pathways occurring on µs or shorter timescales. Existing diagnostic approaches provide a subset of chemical and thermodynamic information. Here we optimize across many diagnostic objectives by introducing a high-speed and broadband, mid-infrared dual-frequency-comb absorption spectrometer. The optical bandwidth of >1000cm−1 covers absorption fingerprints of many species with spectral resolution <0.03cm−1 to accurately discern their absolute quantities. Key to this advance are 1 GHz pulse repetition rate mode-locked frequency combs covering the 3–5 µm region that enable a spectral acquisition rate of 290cm−1 per 17.5 µs per detector for in situ tracking of fast chemical process dynamics. We demonstrate this system to quantify the abundances and temperatures of each species in the complete reactants-to-products breakdown of 1,3,5-trioxane, which exhibits a formaldehyde decomposition pathway that is critical to modern low-temperature combustion systems. By maximizing the number of observed species and improving the accuracy of temperature and concentration measurements, this spectrometer provides a pathway for modern chemistry approaches such as combining chemical models with machine learning to constrain or predict complex reaction mechanisms and rates.