Transient infrared absorption spectra of reaction intermediates detected with a step-scan fourier-transform infrared spectrometer

Our group has utilized a step-scan FTIR spectrometer operating in the absorption mode to characterize transient species in chemical reactions upon photo-irradiation of gaseous mixtures in a multipass White cell. The operational temporal resolution is typically 1-10 μs with spectral resolution 0.1-4 cm^-1, depending on conditions. The acquisition of both ac- and dc-coupled signals enables an extraction of minute changes in the large background signal to attain a typical detectable absorbance variation greater than 1 × 10^-4. By consideration of reaction mechanisms and comparison of vibrational wave numbers, IR intensities, and rotational contours predicted with theoretical calculations, we have assigned IR absorption bands of many important atmospheric free radicals and unstable species including their conformers, such as ClCO, ClSO, ClCS, ClCOOH, CH₃SO₂, CH₃SOO, CH₃OSO, CH₃SO, CH₃OO, CH₃C(O)OO, CH₂OO C₆H₅CO, C ₆H₅SO₂, and C₆H₅C(O)OO. The advantages and limitations of this technique to perform spectral and kinetic investigations of transient species in chemical reactions are discussed.