Reaction kinetics using optical detection

Measuring kinetics of reactive intermediates, such as radicals, is often difficult because of a lack of adequate detection techniques and because such species typically have short lifetimes and low steady state concentrations.

The ideal experimental detection technique for reaction kinetics must meet the following requirements:

  • Non-intrusive probing of the reaction volume: Homogeneity of the sample is crucial to reliable measurements of reaction kinetics.
  • Direct time-resolved probing: Measuring the time history of a species is the simplest way to extract rate coefficients.
  • High sensitivity: Radical-radical reactions are typically very rapid. To avoid secondary chemistry, reacting samples must be very dilute, which requires a sensitive probe method.
  • Multiplexed detection: Probing many chemical species at the same time reduces systematic errors in measurements.

All of the above requirements are met when using time-resolved, broadband, cavity-enhanced absorption spectroscopy (TR-BB-CEAS) based detection. In our group, we have constructed a TR-BB-CEAS apparatus designed for temperature- and pressure-dependent kinetic measurements of atmospherically important stabilized Criegee Intermediates (sCIs) [1].

TR-BB-CEAS apparatus

The main components of the TR-BB-CEAS apparatus are a broadband optical cavity, a slow-flow chemical reactor, and a novel spectrometer. The high sensitivity of the method is due to the high UV absorption cross section of sCIs and the broadband optical cavity, which is capable of achieving long effective optical path lengths. The second component is a flow reactor tube, which is combined with the optical cavity enabling photolytic initiation of gas phase chemical reactions in a homogeneous environment with well-defined conditions: temperature, pressure, and mixture composition. The remaining important experimental component is a novel grating spectrometer, which is designed to disperse the cavity output in wavelength and time domains and to record the entire absorption spectrum of the reacting gas mixture as a function of kinetic time (multiplex detection).


Figure 1. Schematic figure of the time-resolved, broadband, cavity-enhanced absorption spectrometer. The sCI is produced along a heated or cooled flow tube reactor by a single-pass photolysis laser pulse at 213 nm. The sCI is probed by overlapping incoherent laser-driven broadband light source. The sensitivity of the detection is enhanced using an optical cavity formed by two highly reflecting concave mirrors between 300 and 450 nm. The time-dependent broadband absorption spectrum of [sCI] is measured by a grating spectrometer combined with a fast CMOS line array camera.


  1. Jari Peltola, Prasenjit Seal, Niko Vuorio, Petri Heinonen, and Arkke Eskola: Solving the discrepancy between the direct and relative-rate determinations of unimolecular reaction kinetics of dimethyl-substituted Criegee intermediate (CH3)2COO using a new photolytic precursor. Phys. Chem. Chem. Phys., 2022, 24, 5211–5219.
  2. Jari Peltola, Prasenjit Seal, Anni Inkilä and Arkke Eskola: 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., 2020, 22: 11797 – 11808.