This project is a joint experimental and theoretical research program addressing important questions in the areas of high-resolution molecular spectroscopy and collision dynamics. The spectroscopic studies involve measurements of the ro-vibrational energy levels of triplet states of NaCs, and other heteronuclear alkali molecules, and the investigation of fine and hyperfine structure of these levels. The goal is to map out potential energy curves, study spin-orbit and nonadiabatic coupling between states, and to use the pattern of energy levels, particularly the level-by-level changes in the fine and hyperfine structure, to infer fundamental molecular interactions. By comparing data with theoretical models, we have already identified many important dynamical effects. The NaK molecule, in particular, exhibits a fortunate combination of properties that make it an intriguing "laboratory" for the study of molecular hyperfine structure. For example, our recent observations have revealed that different electronic states (and even different ro-vibrational levels within the same state) exhibit several different angular momentum coupling schemes, including both pure and intermediate cases. These level-to-level variations in the fine and hyperfine structure contain information about subtle changes in the electronic wave functions as a function of internuclear separation. Experimental and theoretical techniques have been developed to extract this information. Further high-resolution studies will be done to probe vibrational levels closer to the dissociation limit; data in this range will provide information about long range interactions and will lead to a better understanding of the delicate changes in the electronic wave function as the system makes the transition from the separated atom to the molecular regime. We will also study NaCs, which is of interest for cooling and trapping and for proposed quantum computing schemes. The large spin-orbit interactions in this molecule, leading to strong perturbations and avoided crossings, provide interesting new challenges for both experiment and theory. Our program also studies a variety of collision processes including excitation transfer, transfer of molecular orientation, and velocity changes in collisions that change atomic hyperfine level or molecular rovibrational level. Such studies will provide stringent tests of atomic and molecular collision theory.
Broader impacts are that spectroscopic studies of heteronuclear molecules such as NaK and NaCs are of wide interest. Ultracold heteronuclear alkali diatomics, which have permanent dipole moments, can in principle be oriented in an optical lattice, suggesting applications in quantum computing schemes. Na-Cs and Rb-Cs mixtures are being used in mixed species atom traps, and knowledge of the molecular electronic states is of particular interest for understanding photoassociation spectra. Improved understanding of atomic and molecular collision dynamics will lead to progress in modeling a wide range of environments, from high temperature plasmas to the ultracold regime. The work will contribute to the education of many students. Three or four graduate students will write Ph.D. dissertations based on the projects described in this proposal. At least six undergraduate students will work with the co-PIs during the summers through Lehigh University's REU program. Over the past 25 years, 60 undergraduate students have worked with the co-PIs on various research projects. More than half of these students are female and six belong to underrepresented minority groups; that pattern is likely to continue. The co-PIs will continue to send both graduate and undergraduate students to the APS DAMOP and other national meetings.
