Broadband Time-Resolved Absorption and Dispersion Spectroscopy of Methane and Ethane in a Plasma Using a Mid-Infrared Dual-Comb Spectrometer

Abstract

Conventional mechanical Fourier Transform Spectrometers (FTS) can simultaneously measure absorption and dispersion spectra of gas-phase samples. However, they usually need very long measurement times to achieve time-resolved spectra with a good spectral and temporal resolution. Here, we present a mid-infrared dual-comb-based FTS in an asymmetric configuration, providing broadband absorption and dispersion spectra with a spectral resolution of 5 GHz (0.18 nm at a wavelength of 3333 nm), a temporal resolution of 20 μs, a total wavelength coverage over 300 cm−1 and a total measurement time of ~70 s. We used the dual-comb spectrometer to monitor the reaction dynamics of methane and ethane in an electrical plasma discharge. We observed ethane/methane formation as a recombination reaction of hydrocarbon radicals in the discharge in various static and dynamic conditions. The results demonstrate a new analytical approach for measuring fast molecular absorption and dispersion changes and monitoring the fast dynamics of chemical reactions over a broad wavelength range, which can be interesting for chemical kinetic research, particularly for the combustion and plasma analysis community.

Laser absorption spectroscopy for plasma-assisted thermochemical treatment. Part I.: Applicability of the Beer–Lambert law and interpretation of spectroscopic data

“…Due to different shape of the line strength dependence on temperature, the monitoring of multiple spectral lines for each species will provide additional information for interpretation of the absorption data. Simultaneous recording of a large spectral range with high spectral resolution can be realized with a technique based on a frequency comb [49,50], which provides new opportunities and new challenges for absorption spectroscopy.…”

Optical Frequency Combs for Molecular Spectroscopy, Kinetics, and Sensing

“…Several research groups are actively applying FCS to gas-phase chemical reaction kinetics, harnessing both dual-comb and spatially dispersive methods to study reacting systems in room temperature flow cells, 56,58,[108][109][110][111][112][113] and high temperature flames, discharges, and shock tubes. [114][115][116][117][118][119][120][121][122] The application of these methods to problems in atmospheric chemistry is particularly compelling. One recent room temperature FCS flow cell study by Luo et al concerns the formation and subsequent reaction of the simplest Criegee intermediate, CH 2 OO.…”

Mid-infrared supercontinuum-based Fourier transform spectroscopy for plasma analysis

“…The mid-infrared (MIR) wavelength region (2.5–25 µm) is of particular interest to the spectroscopy community, as many molecular gas species exhibit strong and unique absorption features in this region. Therefore, several different types of MIR sources are used for plasma diagnostics, such as MIR thermal sources (lamps) 15 , 16 , tunable diode lasers 14 , Quantum Cascade Lasers (QCLs) 17 – 19 , Interband Cascade Lasers (ICLs) 20 and optical frequency combs 21 , 22 . While narrowband lasers, i.e., tunable diode lasers, ICLs, and QCLs, are well suited for sensitive detection of a specific molecule, they lack the ability to easily detect different types of molecules simultaneously.…”