Photo-induced Dissociation Dynamics of atmospherically Significant Criegee Intermediates

Alkene ozonolysis, which proceeds through energized carbonyl oxide species also known as Criegee intermediates, is an important oxidation process for atmospheric alkenes and a significant source of hydroxyl (OH) radicals in the troposphere. Criegee intermediates (CH2OO, CH3CHOO, (CH3)2COO) are synthesized by the reaction of iodoalkyl radicals with molecular oxygen in a quartz capillary reactor, cooled in a free jet expansion, and characterized through ultraviolet (UV) and/or infrared (IR) induced dynamical studies. The dissociation dynamics of CH2I2 is investigated using velocity map imaging (VMI) to obtain the velocity distribution of the iodine atom products. The corresponding high internal excitation of the CH2I co-fragments provides insight into the internal excitation of newly formed CH2OO in the subsequent thermo-neutral CH2I + O2 reaction. UV excitation of CH2OO and CH3CHOO on a very strong pi*← pi transition localized in the carbonyl oxide group is shown to result in O-O bond breakage. Both ground O 3P and excited O 1D state products are characterized using VMI to obtain the angular and velocity distributions of the O-atom products. Anisotropic angular distributions of the O-atom products show the rapid nature of the UV photodissociation dynamics. The total kinetic energy distributions reveal the energy required for dissociation into two spin-allowed channels as well as the high degree of internal excitation of the co-fragments. Finally, IR activation of syn-CH3CHOO and (CH3)2COO is utilized to access the barrier to 1,4-hydrogen transfer, which initiates unimolecular decay to OH radical products. The OH fragments are examined using a novel implementation of VMI based on UV+VUV ionization. IR excitation of syn-CH3CHOO and (CH3)2COO in the CH stretch overtone region results in an isotropic angular distribution of OH fragments, indicating that dissociation occurs more slowly than the rotational period of Criegee intermediates. The OH products are released with little internal excitation, while the total kinetic energy release demonstrates that most of the available energy flows into internal excitation of the vinoxy or 1-methlvinoxy co-fragments. The experimental results are compared with quasi-classical trajectory calculations initiated at critical configurations along the reaction pathway and statistical Prior distributions.