Time-Resolved Broadband Cavity-Enhanced Absorption Spectroscopy behind Shock Waves

Abstract

A fast and sensitive broadband absorption technique for measurements of high-temperature chemical kinetics and spectroscopy has been developed by applying broadband cavity-enhanced absorption spectroscopy (BBCEAS) in a shock tube. The developed method has effective absorption path lengths of 60-200 cm, or cavity enhancement factors of 12-40, over a wavelength range of 280-420 nm, and is capable of simultaneously recording absorption time profiles over an ∼32 nm spectral bandpass in a single experiment with temporal and spectral resolutions of 5 μs and 2 nm, respectively. The accuracy of the kinetic and spectroscopic measurements was examined by investigating high-temperature reactions and absorption spectra of formaldehyde behind reflected shock waves using 1,3,5-trioxane as a precursor. The rate constants obtained for the thermal decomposition reactions of 1,3,5-trioxane (to three formaldehyde molecules) and formaldehyde (to HCO + H) agreed well with the literature data. High-temperature absorption cross sections of formaldehyde between 280 and 410 nm have been determined at the post-reflected-shock temperatures of 955, 1265, and 1708 K. The results demonstrate the applicability of the BBCEAS technique to time- and wavelength-resolved sensitive absorption measurements at high temperatures.

Thermal Decomposition of Nitromethane and Reaction between CH3 and NO2

“…NO 2 produced from the thermal decomposition of nitromethane was monitored using timeresolved broadband cavity-enhanced absorption spectroscopy (BBCEAS). The shock tube apparatus and the time-resolved BBCEAS method have been fully described elsewhere, 26 and only specific details are presented here. The sample gases used were mixtures of 80−140 ppm nitromethane (>98% purity; Tokyo Chemical Industry; degassed by repetitive freeze− pump−thaw procedures before use) diluted in argon (>99.9999% purity; Taiyo Nippon Sanso).…”

“…NO 2 was detected by its absorption over the wavelength range 390−410 nm, and the wavelength-averaged absorbance is reported here because there was almost no wavelength dependence in the absorption cross section at high temperatures. The effective absorption path length in the BBCEAS measurement was calibrated using the absorption of NO 2 at room temperature; 26 it was 160−230 cm in the studied wavelength region. The absorption cross section of NO 2 , σ, at high temperature was directly measured by shock-heating mixtures of 100−500 ppm NO 2 diluted in argon.…”

Thermal Decomposition of Benzyl Radicals: Kinetics and Spectroscopy in a Shock Tube

“…The experiments were performed in a shock tube with time-resolved broadband cavityenhanced absorption spectroscopy (BBCEAS), a recently developed technique for recording spectrally resolved transient absorption behind shock waves. 44 The high sensitivity of the cavity-enhanced method enables the detection of weak absorption in a longer wavelength region than before, where previously unreported transient behavior was found. The temporal absorption profiles were analyzed using a kinetic model involving potential secondary reactions to determine the rate constants for benzyl decomposition.…”

Time-resolved mid-infrared dual-comb spectroscopy

“…They can provide an ultrabroadband time-resolved spectrum, but they need a long averaging time to achieve a high signalto-noise ratio (SNR) and detection sensitivity. Two widely used methods are step-scan mechanical Fourier transform spectroscopy (FTS) [1][2][3] and dispersion-based detection [4][5][6][7] . The former exhibits very long measurement times due to the step-scanning, while the latter yields shorter measurement times, but usually has a coarse spectral resolution.…”