Research in the Liu Group at the University of Louisville (UofL) consists of spectroscopic studies on small (but not simple!) molecules using high-resolution laser systems. Our high-resolution laser-spectroscopy studies center on the detection and characterization of reactive chemical intermediates, e.g., free radicals and molecules in excited electronic states. The spectroscopic techniques employed in our experiments include laser-induced fluorescence / dispersed fluorescence (LIF/DF) and cavity ring-down (CRD) spectroscopy. We are also developing a novel cavity-enhanced two-photon double-resonance spectroscopy technique with our state-of-the-art laser systems to study molecular dark states. These studies lead to a quantitative understanding of molecular structure and dynamics and the nature of chemical bonding. Unambiguous identification and analysis of the experimentally obtained spectra is a prerequisite to subsequent work on the chemical reactions that involve chemical reaction intermediates. Recently, we have investigated metal-containing molecules that are promising candidates for direct laser cooling and the search for new physics. We are particularly interested in molecular species with the Jahn-Teller (JT) effect and the pseudo-Jahn-Teller (pJT) effect, vibronic (vibrational-electronic) interactions that cause spontaneous distortion of the symmetry of polyatomic molecules. Our group also develops spectroscopic models to analyze the rotational and fine structure of open-shell molecules (free radicals) in degenerate and nearly degenerate states subject to JT, pJT, and spin-orbit (SO) interactions. Quantum chemistry calculations using the density functional theory (DFT), complete active space self-consistent field (CASSCF), and coupled-cluster (CC) methods are used to help understand the energy level structure of molecules and to predict and simulate experimentally obtained spectra. 

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The goal of the ultrafast spectroscopy study in our group, which is carried out in the Conn Center for Renewable Energy Research, is to contribute to renewable energy science and technology through fundamental spectroscopic studies using ultrafast laser systems. Femtosecond (fs) transient absorption (TA) spectroscopy and time-resolved photoluminescence spectroscopy are used in our lab. Our research efforts in the field of ultrafast spectroscopy are focused on the characterization of solar cell materials, especially nanostructures and polymers. We aim to understand the fundamental photoinduced processes in the materials and their interfaces, which are essential to their functions and applications.

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" Spin-vibronic interactions in asymmetric-top open-shell molecules and their effects on vibrational and rotational branching ratios", presented at the 2022 "Laser-Cooling of Molecules" Workshop of the Institute for Theoretical Atomic, Molecular and Optical Physics (ITAMP) at the Harvard & Smithsonian Center for Astrophysics (CfA). 

Molecular Spectroscopy and the continuous-wave optical parametric oscillator (CW OPO) system from Toptica Photonics, Interview with Dr. Jinjun Liu

WHAS-TV reporting UofL Laser Labs:

Jahn-Teller distortion and pseudo-rotation. Molecular physics is fun!