Time-resolved broadband cavity-enhanced absorption spectroscopy for chemical kinetics.

Abstract

Experimental measurements of elementary reaction rate coefficients and product branching ratios are essential to our understanding of many fundamentally important processes in Combustion Chemistry. However, such measurements are often impossible because of a lack of adequate detection techniques. Some of the largest gaps in our knowledge concern some of the most important radical species, because their short lifetimes and low steady-state concentrations make them particularly difficult to detect. To address this challenge, we propose a novel general detection method for gas-phase chemical kinetics: time-resolved broadband cavity-enhanced absorption spectroscopy (TR-BB-CEAS). This all-optical, non-intrusive, multiplexed method enables sensitive direct probing of transient reaction intermediates in a simple, inexpensive, and robust experimental package. 4 ACKNOWLEDGMENTSWe thank Dr. David Osborn (8353) and Dr. Steven S. Brown (NOAA, Boulder, CO) for helpful discussions regarding cavity-enhanced optical methods and experimental design. We also thank Dr. Craig Taatjes (8353) for the use of lab space and for the loan of a YAG laser and optics. 5

Direct kinetic measurements and theoretical predictions of an isoprene-derived Criegee intermediate

“…MVK-oxide spectroscopy. The MVK-oxide electronic spectra recorded by direct absorption at 298 K using both the Sandia (34) and Institute of Atomic and Molecular Sciences (IAMS) (35) experimental apparatus span the same spectral range (300 to 450 nm) as that reported recently by Vansco et al (31) using a depletion method under jet-cooled conditions (Fig. 1).…”

“…The present study builds on the work of Vansco et al (31) and reports direct absorption spectra of MVK-oxide at 298 K, obtained using two separate experiments (33)(34)(35). Using the characterized direct UV-Vis absorption spectra, we have employed broadband absorption spectroscopy to conduct direct bimolecular kinetics studies of MVK-oxide with key tropospheric species: SO 2 , formic acid, and water.…”

Time-resolved, broadband UV-absorption spectrometry measurements of Criegee intermediate kinetics using a new photolytic precursor: unimolecular decomposition of CH2OO and its reaction with formic acid

“…1. The design of our TR-BB-CEAS apparatus is a modified version of the experimental setup developed by L. Sheps et al 31 The gas mixture flowing through a quartz tube reactor contained the radical precursor (CH 2 IBr or CH 2 I 2 ), O 2 , and HCOOH (for the bimolecular reaction) diluted in helium or nitrogen carrier gas. Calibrated mass flow controllers (Omega Engineering) were used to deliver gases to a mixing manifold before flowing into the reactor.…”

“…The camera (1 Â 2048 pixels, 12-bit) has a maximum line rate of 80 kHz, however, to gain good enough signal-to-noise ratio (SNR), the line rate was reduced to 20 kHz leading to time resolution of about 50 ms. The use of the fast line array camera enables the transient spectrum measurements without moving parts (e.g., a spinning mirror 31,32 ) inside the grating spectrometer. All the transient absorption traces of CH 2 OO measured in the current work were probed at 340 nm region, where the strong absorption band of CH 2 OO has its maximum and the effective path length about 80 m. This region is also free from unwanted interferences of byproducts, e.g.…”

The reaction of Criegee intermediate CH2OO with water dimer: primary products and atmospheric impact

“…Constant T and P in the reactors are maintained by feedback-controlled resistive heaters and downstream throttle valves; sample composition is regulated by precision flow controllers. The TR-BB-CEAS apparatus 35,36 is a 1.6 m-long broadband optical cavity that uses continuous "white-light" probe radiation from a Xe arc lamp and operated at probe λ = 300 -450 nm simultaneously for this study. The cavity is integrated into a quartz laser photolysis flow reactor with ID = 3 cm, capable of operating up to 650 K and 500 Torr.…”