Competing Pathways in Reactions of Oxygen-Containing Free Radicals

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Through this award, funded by the Chemical Structure, Dynamics, and Mechanisms - A Program of the Division of Chemistry, Prof. H. Floyd Davis and students at Cornell University will study photoinduced and bimolecular reactions involving atmospherically important oxygen-containing free radicals. Using the crossed laser- molecular beam method, the branching ratios for competing bromine dioxide and iodine dioxide photodissociation pathways will be determined. Elementary radical-radical reactions involving diatomic halogen oxide free radicals (e.g., ClO + BrO) will be studied in crossed molecular beams. A method involving photodepletion of highly-reactive neutral halogen oxide molecules with near-threshold photoionization detection will be developed to determine wavelength-dependent absorption cross sections for species that cannot be prepared in pure form. The proposed studies will provide experimental benchmarks, including ionization energies, bond dissociation energies, and branching ratios for key atmospheric reactions induced by absorption of solar radiation or bimolecular collisions. These quantities should help facilitate direct tests of quantum theoretical models for reactions, and should be of use in atmospheric models for ozone-depleting processes. Halogen atoms (chlorine, bromine, and iodine) undergo reactions in the Earth's atmosphere that destroy ozone, an important molecule that provides protection from harmful ultraviolet solar radiation. The ozone-destroying reactions involve molecules with unpaired electrons known as free radicals. Because of their extremely reactive nature, most free radicals cannot be prepared in pure form, making their study difficult. In this research project, laboratory techniques involving laser beams (collimated beams of light) and molecular beams (collimated beams of molecules), are used to study chemical reactions involving atmospheric free radicals. In addition, the Davis group will establish a high-school lending program for newly-developed low-cost ($25) absorption spectrometers. This will help bring hands-on laboratory experiments, usually requiring high-cost equipment, to the high school classroom. Exposing high school students to the design, construction, and use of scientific instruments can be used to teach critical thinking skills and help students understand and appreciate science.