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Direct Spectroscopic and Kinetic Studies of the Simplest Criegee Intermediate

  • Author(s): Foreman, Elizabeth Stack
  • Advisor(s): Murray, Craig
  • et al.
Abstract

Atmospheric chemistry is driven largely by free radical species whose reactions determine both the composition and oxidative capacity of the atmosphere. Carbonyl oxides, or Criegee Intermediates, formed in the ozonolysis of alkenes in the troposphere are highly reactive biradicals/zwitterions that are implicated in the production of secondary organic aerosol and particulate matter. This thesis will focus on the following aspects of the spectroscopy and kinetics related to the simplest Criegee Intermediate, formaldehyde oxide: (1) kinetics of IO radical formation from the CH2I + O2 reaction, (2) spectroscopy and dynamics of electronically excited states of CH2OO, (3) the kinetics of reactions of CH2OO with trace atmospheric pollutants. The spectroscopic and kinetic study of IO radicals formed as minor products in the laboratory synthesis of Criegee Intermediates is performed using cavity ring-down spectroscopy (CRDS). We observe experimentally that IO radicals exhibit vibrational state specific formation kinetics and suggest that this is due to a product branching ratio controlled by the degree of internal excitation in the reactants. IO and CH2OO are produced simultaneously in the reaction of CH2I with O2 and exhibit overlapping absorption bands in the near-ultraviolet/visible. This poses a challenge for direct detection of CH2OO using a linear absorption technique. The measured rate constant for the formation of IO is used to deconvolute the total observed absorption spectrum into individual molecular components. Both CRDS and single-pass broadband transient absorption spectroscopy (BBTAS) are used to independently measure the absolute absorption cross sections for the second electronically excited state of CH2OO in the near-ultraviolet/visible. We show that the absorption spectra do not exhibit fine structure, even at the lowest excitation energies, and that the observed broad lineshapes cannot be attributed hot bands. This suggests that the photolysis quantum yield is ≪1 and the observed structure in the electronic absorption spectrum, that is absent from the action spectrum, is the result of relaxation via nonradiative population transfer to a long-lived state or fluorescence. BBTAS is subsequently used to probe the kinetics of the reactions of CH2OO with two inorganic acids: HCl and HNO3. We find that these reactions occur at or near the collision limit and that nitric acid in particular may be locally competitive with water vapor, which is widely accepted as the dominant sink for Criegee Intermediates in the atmosphere. Taken together these studies provide a deeper understanding of the role Criegee Intermediates play in tropospheric chemistry.

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