Toward a deeper understanding of the quantum nature of molecular collisions

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Millard Alexander of the University of Maryland, College Park, is supported by an award from the Chemical Theory, Models and Computational program in the Chemistry division to conduct rigorous theoretical studies, based on quantum mechanics, of chemical collisions, including chemical reactions. The award is cofunded by the Chemical Structures Dynamics and Mechanisms-A (CSDM-A) program, also in the Chemistry Division, and the Theoretical Atomic, Molecular and Optical Physics (TAMOP) program in the Physics division. The essence of chemistry is the rearrangement of atoms from one arrangement to another. A chemical reaction is akin to driving from one valley over a mountain pass into another valley. The pass is called a "transition state". The rate at which "reaction" occurs is controlled by the height of the pass which separates the valleys, the ease of access to the pass, and the speed at which the pass is surmounted. Experiments are limited to a before and after view, monitoring how many automobiles disappear from the initial valley, how many arrive in the new valley, and the velocity of the atoms after reaction. From this, the chemist tries to infer how chemical bonding affects the shape of the mountain pass and, thus, the rate at which chemical change occurs. Theoretical models, of the kind that Alexander develops, provide insight which complements experiment. Alexander's work is particularly focused on how the uncertainty of quantum mechanics influences chemical reactivity. The research has relevance for many fields including astrochemistry, atmospheric chemistry and ultra-cold chemistry. The Alexander group develops and disseminates computer software for use by the research community. Many aspects of the dynamics of small-molecule collisions can be well understood on the basis of classical simulations. Notwithstanding, a quantum treatment provides the only fully correct description of many elementary reactions, especially those involving H atoms; at the fully-resolved state-to-state level; at low temperature; or when the two (or more) electronic potential energy surfaces are accessed simultaneously. In the latter case, exactly as in the paradigm two-slit situation, only quantum mechanics can describe the interference between trajectories which access the distinct potential energy surfaces. To piece out where quantum effects -- resonances, or the breakdown in the Born-Oppenheimer approximation -- show themselves within the vast classical domain of molecular dynamics is the goal of research in the Alexander group. Sophisticated quantum-chemical techniques are used to develop accurate potential energy surfaces for paradigm reactions. These are then used in quantum scattering calculations of cross sections and, ultimately, rate constants. Alexander's work, based substantially on computer codes developed in house, continues to provide insight and understanding for a number of top experimental groups worldwide.