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Polik Research Group

Laser Spectroscopy and Computational Chemistry of Highly Energetic Molecules

This research program is directed toward understanding chemical reactivity by studying the behavior of molecules with large amounts of energy, as it is these molecules which undergo chemical reaction. Highly excited vibrational states of polyatomic molecules are prepared and studied by combining laser spectroscopy, molecular beam techniques and high resolution dispersed fluorescence spectroscopy. The experimental approach is to cool molecules to a few degrees Kelvin in a molecular beam, thereby removing spectral congestion. A narrowband laser is used to selectively excite sample molecules to a single quantum state in the first excited electronic state. The resulting fluorescence is dispersed with a monochromator, yielding a spectrum of highly excited levels in the ground electronic state. The fluorescence emission spectrum contains information about the energy levels and dynamics of the excited states. Results are being analyzed for H2CO and D2CO, and other molecules, e.g., HDCO and DFCO, and various radicals, e.g. H2CCH., are proposed for future study.

A related area of research is the computation of anharmonic potential energy surfaces (PES) and computed vibrational spectroscopy of several molecular systems: H2O/HDO/D2O, H2CO/HDCO/D2CO, HFCO/DFCO and HCO/DCO. High accuracy quantum chemistry programs are used to compute the quartic PES of a molecule by numeric differentiation. The harmonic, anharmonic and resonance constants are determined, from which highly excited vibrational states can be computed and compared to experimental results. The long-range goal is to develop a protocol for computing potential energy surfaces and vibrational states to a prespecified level of accuracy. Students use a high-power computer cluster (100 64-bit CPUs, 400 GB RAM, 6 TB storage) and state-of-the-art computational chemistry programs (aces, gamess, gaussian, molpro, nwchem, qchem) to carry out this work.

This experimental and theoretical research program has proven particularly amenable to effective involvement of undergraduates, with between three and eight students conducting research during the academic year and an average of five students conducting research each summer. Each step of the experiment illustrates important concepts in general chemistry, general physics and physical chemistry lectures. The research equipment and experimental procedures allow students to make major contributions to the study of scientific problems. Specifically, students have "hands-on" experience using Nd:YAG and tunable dye lasers, optics, a monochromator, a molecular beam chamber, glass and metal vacuum systems, optical detection equipment, computer interfaces and electronic signal conditioning equipment. The data analysis requires students to reach far beyond their formal classroom education and refer to specialized textbooks and the chemical literature. Students participate in every aspect of the experiment: design, construction, testing, troubleshooting, data acquisition, data analysis, and oral/written presentations and manuscript preparation. The theoretical program allows students to understand and make predictions about chemical phenomena. Fundamental ideas of physical and quantum chemistry are used, which illustrate principles learned in physical chemistry courses. The entire research group holds weekly meetings in which experimental techniques, computational results, related research in the literature and their results are discussed. In short, students are active participants in all aspects of the scientific method.

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