Abstract
Fast radical reactions are central to the chemistry of planetary atmospheres and combustion systems. Laser-induced fluorescence is a highly sensitive and selective technique that can be used to monitor a number of radical species in kinetics experiments, but is typically limited to low pressure systems owing to quenching of fluorescent states at higher pressures. The design and characterisation of an instrument are reported using laser-induced fluorescence detection to monitor fast radical kinetics (up to 25 000 s(-1)) at high temperatures and pressures by sampling from a high pressure reaction region to a low pressure detection region. Kinetics have been characterised at temperatures reaching 740 K and pressures up to 2 atm, with expected maximum operational conditions of up to ∼900 K and ∼5 atm. The distance between the point of sampling from the high pressure region and the point of probing within the low pressure region is critical to the measurement of fast kinetics. The instrumentation described in this work can be applied to the measurement of kinetics relevant to atmospheric and combustion chemistry.
Highlights
A variety of experimental approaches exist for the extraction of rate coefficients; for radical reactions, real-time measurements monitoring the removal of reagents or generation of products following the production of radicals by laser flash photolysis (LFP) are a successful approach
At a probe distance of 5 mm, the observed OH decay was well-described by pseudo-firstorder kinetics and the bimolecular rate coefficient obtained for OH + Dimethyl ether (DME), kOH+DME = (2.6 ± 0.1) × 10−12 cm3 s−1, is in good agreement with recent determinations of kOH+DME = (2.7 ± 0.2) × 10−12 cm3 s−1 18 and kOH+DME = (2.9 ± 0.3) × 10−12 cm3 s−1 43 and literature recommendations of kOH+DME = (2.8 ± 0.4) × 10−12 cm3 s−1 45 at 298 K
Radical generation in the high pressure reaction region is achieved by laser flash photolysis, minimising the potential for heterogeneous chemistry
Summary
The kinetics of gas phase reactions involving free radicals are central to the chemistry of planetary atmospheres, interstellar space, and combustion systems. A variety of experimental approaches exist for the extraction of rate coefficients; for radical reactions, real-time measurements monitoring the removal of reagents or generation of products following the production of radicals by laser flash photolysis (LFP) are a successful approach. Laser-induced fluorescence (LIF) spectroscopy is commonly employed in flash photolysis experiments to monitor changes in radical concentrations throughout the course of a reaction in real-time, enabling determination of kinetic parameters. We describe the use of LIF at low pressures (∼1 Torr) in the Fluorescence Assay by Gas Expansion (FAGE) technique to determine reaction kinetics of reactive species at higher pressures (>1 atm). Field instruments to measure the total loss rate of OH in the atmosphere (k ′OH, the OH reactivity) are currently in operation which use LFP in conjunction with LIF-FAGE to make real-time measurements of OH radical kinetics with ambient concentrations of trace gases at atmospheric pressure.31–38 These instruments rely on flash photolysis of O3 (followed by the subsequent reaction of O(1D) with water vapour) to produce relatively high concentrations of OH radicals in a reaction cell containing a flow of ambient air at atmospheric pressure. The bimolecular rate coefficient (i.e., kOH+X) is obtained from the gradient of the bimolecular plot (i.e., the plot of the pseudo-first-order rate coefficient against [X], the known concentration of reactant X)
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