A sensitive Cavity Ringdown Spectroscopy (CRDS) apparatus has been successfully developed to study reactive intermediates in the near IR region. Many interesting species can be observed in this chemical fingerprint spectral region. Peroxy radicals (RO2), a class of important intermediates in atmospheric chemistry and low temperature combustion, have been investigated via IR Ã – X̃ electronic transition.
Moderate resolution (0.2 cm−1) absorption spectra of the near IR electronic transition have been obtained for several members of the alkyl peroxy radical family: methyl (CH3O2), ethyl (C2H5O2) and isopropyl (2-C3H7O2) peroxy. CRDS spectra of the acetyl peroxy radical (CH3C(O)O2), a key irritant in smog, has been investigated. A sharp, structured electronic spectrum of à – X̃ electronic transition for the fluorinated methyl peroxy (CF3O2) radical has been observed. Extensive ab initio calculations have been carried out to help assign spectral carrier, predict and analyze observed spectroscopic features such as rotational contours, vibrational hot bands and frequencies for the à state fundamental vibrations. Structured spectral signatures of the transition contain information sufficient to distinguish the detailed nature of the R group, even when they are quite similar, providing an opportunity to easily identify different peroxy radicals and develop species specific diagnostics. The absorption cross sections for several peroxy radicals has been determined using CRDS observations and radical recombination kinetic measurements.