Millard Alexander of the University of Maryland, College Park is supported by an award from the Theoretical and Computational Chemistry program within the Division of Chemistry for the development of theoretical tools to investigate, with high accuracy, reactions occurring on multiple coupled potential energy surfaces. Fully-quantum reactive scattering calculations are used to study the effects of spin-orbit branching in the products of elementary reactions. Among the reactions studied are several of atmospheric interest: the reactions of atomic oxygen and atomic fluorine with hydrogen chloride. This work is crucial to interpreting and challenging a large number of existing, detailed experimental investigations.
The research has a broad impact through application to atmospheric, combustion and interstellar chemistry and involves international collaborations with leading theoretical and experimental chemists from Europe and Asia. Alexander also freely distributes several computer codes developed in his research group.
Intellectual Merit Millard Alexander has investigated how chemical reactions occur on multiple potential energy surfaces (PES’s), with a particular focus on elementary reactions of wide experimental interest. His previous work on inelastic collisions of open-shell molecules and on the paradigm halogen-hydrogen exchange reactions has transformed our ability to understand, both qualitatively and quantitatively, how the overall reactivity depends on the different electronic states of the reactants and products. The unique methodology developed by Alexander is crucial for interpreting and continues to challenge ongoing experimental investigations. Alexander is applying his expertise to more complex reactions involving two halogen atoms [F(2P)+HCl→HF+Cl] as well as to non-Born-Oppenheimer quenching in the important hydroxyl radical. This work is contributing to an understanding of what controls spin-orbit branching in products of elementary reactions. In addition, Alexander has investigated quantum effects in low-energy collisions of small molecules, under experimental study in several labs in Europe and of great astrophysical importance. The intellectual merit of the proposed research is that at present Alexander's methodology provides the only way to carry out accurate state-to-state calculations on the dynamics of chemical reactions involving multiple PES’s. In addition, Alexander is developing novel methods for the quantum treatment of this dynamics and for visualizing the results. Broader impact The broader impact of the proposed research extends to both the scientific and wider educational communities. Molecular free radicals play a key role in many chemical environments, including combustion, propulsion, materials processing, plasmas, and remediation chemistry. Alexander's work on the breakdown in the Born-Oppenheimer approximation in elementary gas-phase processes, both reactive and non-reactive, will provide a foundation for the deeper understanding of a large number of recent experiments, as well as for important applied chemical processes in the atmosphere, in interstellar gas clouds, and in low-temperature laboratory experiments. Alexander’s work involves a number of long-standing national and international collaborations, in particular with groups at Johns Hopkins and Stuttgart. The results of the proposed research will be disseminated across the scientific community. In addition, Alexander will continue the free distribution of computer codes and advanced instructional modules in collision dynamics and quantum chemistry developed in his group. Alexander has also mentored a significant number of undergraduate and graduate students, as well as postdoctoral fellows, in physical chemistry.
