Note: a transient absorption spectrometer using an ultra bright laser-driven light source

A white cell based broadband transient UV-vis absorption spectroscopy with pulsed laser photolysis reactors for chemical kinetics under variable temperatures and pressures

UV-vis spectroscopy is widely used for kinetic studies in physical chemistry, as species' absolute cross-sections are usually less sensitive to experimental conditions (i.e., temperature and pressure). Here, we present the design and characterization of a multipass UV-vis absorption spectroscopy white cell coupled to a pulsed-laser photolysis flow reactor. The glass reactor was designed to facilitate studies of gas phase chemical reactions over a range of conditions (239-293 K and 10-550 Torr). Purged windows mitigate contamination from chemical precursors and photolysis products. We report the measured impact of this purging on temperature uniformity and the absorption length and present some supporting flow calculations. The combined optical setup is unique and enables the photolysis laser to be coaligned with a well-defined absorption pathlength probe beam. This alignment leverages the use of one long-pass filter to increase the spectrum flatness and increase the light intensity vs other systems that use two dichroic mirrors. The probe beam is analyzed with a dual exit spectrograph, customized to split the light between an intensified CCD and photomultiplier tube, enabling simultaneous spectrum and single wavelength detection. This multipass system yields a pathlength of ∼450 cm and minimum observable concentrations of ∼3.7 × 1011 molecule cm-3 (assuming cross-sections ∼1.2 × 10-17 cm2). The temperature profile across the reaction region is ±2 K, defined by the worst-case temperature of 239 K, validated by measurements of the N2O4 equilibrium constant. Finally, the system is implemented to study the simplest Criegee intermediate, demonstrating the instrument performance and advantages of simultaneous spectrum and temporal profile measurements.

High-sensitivity electronic Stark spectrometer featuring a laser-driven light source

We report developmental details of a high-sensitivity Stark absorption spectrometer featuring a laser-driven light source. The light source exhibits intensity fluctuations of ∼0.3% over timescales ranging from 1 min to 12 h, minimal drift (≤0.1%/h), and very little 1/f noise at frequencies greater than 200 Hz, which are comparable to or better than an arc-driven light source. Additional features of the spectrometer include balanced detection with multiplex sampling, which yielded lower noise in A, and constant wavelength or wavenumber (energy) spectral bandpass modes. We achieve noise amplitudes of ∼7 × 10-4 and ∼6 × 10-6 in measurements of single A and ΔA spectra (with 92 data points) taking ∼7 and ∼19 min, respectively.

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

Isoprene has the highest emission into Earth's atmosphere of any nonmethane hydrocarbon. Atmospheric processing of alkenes, including isoprene, via ozonolysis leads to the formation of zwitterionic reactive intermediates, known as Criegee intermediates (CIs). Direct studies have revealed that reactions involving simple CIs can significantly impact the tropospheric oxidizing capacity, enhance particulate formation, and degrade local air quality. Methyl vinyl ketone oxide (MVK-oxide) is a four-carbon, asymmetric, resonance-stabilized CI, produced with 21 to 23% yield from isoprene ozonolysis, yet its reactivity has not been directly studied. We present direct kinetic measurements of MVK-oxide reactions with key atmospheric species using absorption spectroscopy. Direct UV-Vis absorption spectra from two independent flow cell experiments overlap with the molecular beam UV-Vis-depletion spectra reported recently [M. F. Vansco, B. Marchetti, M. I. Lester, J. Chem. Phys. 149, 44309 (2018)] but suggest different conformer distributions under jet-cooled and thermal conditions. Comparison of the experimental lifetime herein with theory indicates only the syn-conformers are observed; anti-conformers are calculated to be removed much more rapidly via unimolecular decay. We observe experimentally and predict theoretically fast reaction of syn-MVK-oxide with SO₂ and formic acid, similar to smaller alkyl-substituted CIs, and by contrast, slow removal in the presence of water. We determine products through complementary multiplexed photoionization mass spectrometry, observing SO₃ and identifying organic hydroperoxide formation from reaction with SO₂ and formic acid, respectively. The tropospheric implications of these reactions are evaluated using a global chemistry and transport model.

Synthesis, Electronic Spectroscopy, and Photochemistry of Methacrolein Oxide: A Four-Carbon Unsaturated Criegee Intermediate from Isoprene Ozonolysis

Ozonolysis of isoprene, one of the most abundant volatile organic compounds in the earth's atmosphere, generates the four-carbon unsaturated methacrolein oxide (MACR-oxide) Criegee intermediate. The first laboratory synthesis and direct detection of MACR-oxide is achieved through reaction of photolytically generated, resonance-stabilized iodoalkene radicals with oxygen. MACR-oxide is characterized on its first π* ← π electronic transition using a ground-state depletion method. MACR-oxide exhibits a broad UV-visible spectrum peaked at 380 nm with weak oscillatory structure at long wavelengths ascribed to vibrational resonances. Complementary theory predicts two strong π* ← π transitions arising from extended conjugation across MACR-oxide with overlapping contributions from its four conformers. Electronic promotion to the 1¹ππ* state agrees well with experiment, and results in nonadiabatic coupling and prompt release of O ¹D products observed as anisotropic velocity-map images. This UV-visible detection scheme will enable study of its unimolecular and bimolecular reactions under thermal conditions of relevance to the atmosphere.

Strong Negative Temperature Dependence of the Simplest Criegee Intermediate CH2OO Reaction with Water Dimer

The kinetics of the reaction of CH2OO with water vapor was measured directly with UV absorption at temperatures from 283 to 324 K. The observed CH2OO decay rate is second order with respect to the H2O concentration, indicating water dimer participates in the reaction. The rate coefficient of the CH2OO reaction with water dimer can be described by an Arrhenius expression k(T) = A exp(-Ea/RT) with an activation energy of -8.1 ± 0.6 kcal mol(-1) and k(298 K) = (7.4 ± 0.6) × 10(-12) cm(3) s(-1). Theoretical calculations yield a large negative temperature dependence consistent with the experimental results. The temperature dependence increases the effective loss rate for CH2OO by a factor of ~2.5 at 278 K and decreases by a factor of ~2 at 313 K relative to 298 K, suggesting that temperature is important for determining the impact of Criegee intermediate reactions with water in the atmosphere.

The UV absorption spectrum of the simplest Criegee intermediate CH2OO

The absolute UV absorption spectrum of CH₂OO was determined using multiple methods.