Time-resolved, Broadband UV-Absorption Spectrometry Measurements of Criegee Intermediate Kinetics using a New-Type Photolytic Precursor

Abstract:

Very recently, we introduced a new time-resolved, broadband cavity-enhanced UV-absorption spectrometer apparatus (see Fig. 1) that we have constructed and utilized for temperature- and pressure-dependent measurements of stabilized Criegee Intermediate (sCI) kinetics [1]. In that study, we showed the capability of the new set-up for unimolecular decomposition kinetic measurements of formaldehyde oxide (CH₂OO) and its reaction with formic acid over wide pressure and temperature ranges. We also introduced a new photolytic precursor for CH₂OO, bromoiodomethane (CH₂IBr), which photolysis at 213 nm in presence of O₂ produces CH₂OO. This new precursor was found to be free from secondary reactions that may regenerate CH₂OO in kinetic experiments.

Here, we present our latest results of unimolecular decomposition kinetic study of acetone oxide ((CH₃)₂COO) using 2-bromo-2-iodopropane ((CH₃)₂CIBr) as precursor for (CH₃)₂COO. The unimolecular decomposition rate coefficient of (CH₃)₂COO has been measured over wide pressure (5–350 Torr) and temperature (296–340 K) ranges. We compare our results with the outcome of earlier unimolecular decomposition measurements of (CH₃)₂COO by Smith et al. [2] as well as with results of theoretical studies from literature.

Figure 1 (below): A schematic picture of the time-resolved, broadband cavity-enhanced absorption spectrometer. The sCI is produced along a heated (cooled) quartz flow tube reactor by a single-pass photolysis laser pulse. The formed sCI is probed by overlapping incoherent broadband beam. The sensitivity of the detection is enhanced using an optical cavity formed by two highly reflecting concave mirrors. The time-dependent broadband absorption spectrum of sCI is measured by a grating spectrometer combined with a fast CMOS line array camera.

• Peltola, et al., “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,” Phys. Chem. Chem. Phys. 22, pp. 11797–11808, 2020.
• C. Smith, et al., “Unimolecular decomposition rate of the Criegee intermediate (CH₃)₂COO measured directly with UV absorption spectroscopy,” J. Phys. Chem. A 120, pp. 4789–4798, 2016.