Cavity ringdown spectroscopy of atmospherically important radicals

Abstract

Many radicals, due to their electronic structure, have low-lying electronic states transitions to which lie in the near-IR. They often carry more information about the molecules than the transitions in the UV. However, these transitions even in the most important atmospheric radicals have not been thoroughly investigated due to their weakness and low attainable concentrations of radicals. This thesis describes the application of cavity ringdown spectroscopy to detection of near-IR states of some atmospherically important radicals. The near-IR cavity ringdown spectrometer constructed for these experiments is described in detail and characterized. The pulsed near-IR laser radiation was generated by sequential Raman shifting of the output of a tunable dye laser in hydrogen. The constructed multi-pass Raman cell extended the tunable range of the available dye laser continuously from the visible to 6000 cm⁻¹ with 0.15 cm⁻¹ resolution. The sensitivity of the instrument is ≈0.5 % of the mirror loss. The near-IR A ← X~ transition in peroxy radicals offers detection specificity for at least small radicals. The sensitivity of this transition to hydrogen atom substitution has been explored. The spectra of chloro-ethyl, -propyl, -butyl and -butenyl peroxy radicals in the 7000-8600 cm⁻¹ region are reported. The origin bands of the electronic transition were found to be shifted by 200 cm⁻¹ to the red. The spectra have more complex structure than those of unsubstituted homologues. DFT calculations predicted multiple conformers for C₂H₄ClO₂ and C₃H₆ClO₂ with the energies within 2 kcal/mol. Tentative assignment of the C₂H₄ClO₂ spectrum is presented. The integrated cross-section for the transition in chloro-ethyl peroxy radical is estimated from the known rate of self-reaction. The first full absorption spectrum of the dark A ²E' ← X~ ²A'₂ transition of the nitrate radical NO₃ in the 6000-10700 cm⁻¹ region is reported. ν₂, ν₃, and ν₄ progressions and several combination bands are assigned. A more accurate estimate for the position of the dark origin is given. Analysis of the partially resolved rotational contours suggests that NO₃ undergoes static Jahn-Teller distortion in some of the vibronic states.