High-Resolution Molecular Spectroscopy on Multiple Potential Energy Surfaces using Cavity-Ring-Down-Based Two-Photon Techniques

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In this project funded by the Chemical Structure, Dynamics, and Mechanisms A (CSDM-A) program of the Chemistry Division, Professor Jinjun Liu and his research team at the University of Louisville (UofL) are using sophisticated laser techniques to study the complex motions of nuclei and electrons in molecules, including electrons’ “orbiting” around the nuclei, the spin of electrons, and the vibrational and rotational motions of nuclei. In a molecule, these motions are “quantized”, meaning that they are not continuous but discrete. As a result, the molecule can be on separated “energy levels” with different energies and “quantum states”. Understanding the energy-level structure, and the distribution and transfer of energy between energy levels is essential to many disciplines of physical sciences. However, experimentally obtained data on the energy level structure of molecules are often extremely complicated and difficult to understand because different motions of electrons and nuclei coincide and are coupled to each other. Moreover, many quantum states are “dark”, i.e., they cannot be accessed from the lowest-energy state of the molecule using a single laser beam. These two factors often prohibit a comprehensive and quantitative understanding of the molecular energy level structure. The UofL team plans to overcome both obstacles by developing new spectroscopy techniques that use not one, but two laser beams to interrogate molecules. Using two laser beams provides the selectivity on energy levels that is necessary to simplify experimental data. It can also create “detours” to the dark states, given that intermediate quantum states are judiciously selected. In the experiment, the laser light is trapped between two mirrors that form an optical cavity, which significantly increases the time of interaction between the laser and molecules, and hence improves the detection sensitivity. The first molecules that the UofL team studies are nitrogen dioxide (NO2) and the nitrate radical (NO3). Both molecular species play important roles in the chemistry of the atmosphere. NO2 is one of the major pollutants of photochemical air pollution, while NO3 is the primary oxidant in the night-time troposphere. The UofL team is also collaborating with computational chemists to design their experiments and to interpret the expected experimental data.The project focuses on developing two high-resolution, high sensitivity spectroscopic techniques: double-resonance cavity ring-down (DR-CRD) and stimulated-emission pumping cavity ring-up (SEP-CRU). Both methods are based on the highly sensitive CRD technique. They have advantages associated with two-photon spectroscopy, including simplified spectra, sub-Doppler linewidth, and the capability of accessing molecular dark states, which cannot be accessed using single-photon spectroscopy techniques due to forbidding selection rules, small transition dipole moments, or unfavorable Franck-Condon factors. Quantitative information of vibronic (vibrational-electronic) coupling (e.g., the Jahn-Teller and pseudo-Jahn-Teller effects) and related intramolecular interactions (e.g., the spin-orbit interaction) are being investigated using these two new laser spectroscopic techniques. The first target molecules, NO2 and NO3, are two prototypical molecules for the study of vibronic interactions. They are interrogated with the DR-CRD and SEP-CRU techniques at room temperature and under jet-cooled conditions (T~1 K). With intermediate states for the two-step excitation predicted based on computational chemistry and spectroscopic models, the UofL team investigate the spin-rovibronic (spin-rotational-vibrational-electronic) energy level structure of the ground and low-lying excited electronic states of these two free radicals. Two previously developed spectroscopic models, one for vibronic analysis and the other for rotational analysis, have been combined and employed to simulate and fit the spin-rovibronic structure in experimentally obtained spectra, and to unravel the complex mechanism of vibronic interactions. In terms of scientific Broader Impacts, the experimental techniques and the theoretical model developed in this project are a uniquely effective tool for the study of spectroscopy and dynamics on multiple potential energy surfaces (PESs). The post-doctoral researcher and Ph.D. students engaged in this research project are gaining valuable experience in cutting-edge laser-spectroscopy technology, molecular physics, theoretical chemistry, and computer simulations of experimental data.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.