Simultaneous determination of transient free radicals and reaction kinetics by high-resolution time-resolved dual-comb spectroscopy

Pei-Ling Luo,Er-Chien Horng

doi:10.1038/s42004-020-00353-6

Pei-Ling Luo, Er-Chien Horng

Open Access

https://doi.org/10.1038/s42004-020-00353-6

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Abstract

Quantitative determination of multiple transient species is critical in investigating reaction mechanisms and kinetics under various conditions. Dual-comb spectroscopy, a comb-laser-based multi-heterodyne interferometric technique that enables simultaneous achievement of broadband, high-resolution, and rapid spectral acquisition, opens a new era of time-resolved spectroscopic measurements. Employing an electro-optic dual-comb spectrometer with central wavelength near 3 µm coupled with a Herriott multipass absorption cell, here we demonstrate simultaneous determination of multiple species, including methanol, formaldehyde, HO2 and OH radicals, and investigate the reaction kinetics. In addition to quantitative spectral analyses of high-resolution and tens of microsecond time-resolved spectra recorded upon flash photolysis of precursor mixtures, we determine a rate coefficient of the HO2 + NO reaction by directly detecting both HO2 and OH radicals. Our approach exhibits potential in discovering reactive intermediates and exploring complex reaction mechanisms, especially those of radical-radical reactions.

Highlights

Quantitative determination of multiple transient species is critical in investigating reaction mechanisms and kinetics under various conditions
Sensitive detection of transient species can be achieved by using infrared continuous wave lasers coupled with the multipass absorption cell or wavelength modulation technique[4,5]
Time-resolved spectroscopy based on direct frequency comb spectroscopy (DFCS) methods has been used for monitoring spectral variations under flash photolysis[12], electric discharge[13], laser-induced plasma[14,15], and combustion conditions[16] as well as for studying gas-phase reaction kinetics[17,18,19], protein dynamics[20], and population relaxation processes[21]

Summary

Introduction

Quantitative determination of multiple transient species is critical in investigating reaction mechanisms and kinetics under various conditions. The QCLDCS has been presented with a compact system;[19,20] it may be more suitable for time-resolved measurements at relatively low spectral resolution due to the large comb mode spacing (approximately 0.3 cm−1) of QCLs. To investigate rotationally resolved spectra and study gasphase reaction kinetics with distinguishing probes of multiple species, the spectral resolution of the time-resolved spectrometer must be higher than the observed absorption line width. Dual-comb system with mode-locked or quantum cascade lasers can offer a wide measurement range without wavelength sweep, the system is unable to achieve high spectral resolution (

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