【校级报告】Electronic spectroscopy of jet-cooled calcium monoalkoxide radicals (CaOR): Spin-ro-vibronic structure of nonlinear polyatomics as candidates for direct laser cooling

报告人：Prof. Dr. Jinjun Liu, University of Louisville, U.S.A.   

主持人：杨涛研究员   

时间：2022年1月11日（周二）上午9:00-11:00   

地点：光学大楼B325会议室，腾讯会议ID：467 344 258   

报告人简介：

Dr. Jinjun Liu is an Associate Professor of Chemistry and an Adjunct Professor of Physics at the University of Louisville. He received his B.S. degree in physics at East China Normal University in 1999 and worked for two years in the State Key Laboratory of Precision Spectroscopy. For his Ph.D., he studied Chemical Physics at the Ohio State University in Dr. Terry A. Miller's research group. Jinjun received his Ph.D. degree in 2007 and served as a post-doctoral fellow with Dr. Frederic Merkt at the Swiss Federal Institute of Technology (ETH), Zurich. He joined the Department of Chemistry at the University of Louisville as an Assistant Professor in January 2012. He was promoted to Associate Professor in July 2017, and is expected to be officially promoted to Full Professor in July 2022. He is also the Spectroscopy Theme Leader at the Conn Center for Renewable Energy Research. Dr. Liu received the CAREER Award from the National Science Foundation of the U.S. in 2015, the Flygare Award from the International Symposium on Molecular Spectroscopy in 2017, and the Fundamental Physics Innovation Award from the American Physical Society in 2018.

报告内容简介：

Alkaline earth monoalkoxide (MOR) free radicals are promising candidates for laser-cooling of polyatomic molecules.^[1],[2] The fast advance of laser-cooling of molecules and their broad applications in physical sciences requires more detailed investigation and quantitative understanding of the spin-ro-vibronic (spin-rotational-vibrational-electronic) energy level structure and transition intensities, both by laser-spectroscopy measurements and by first-principles calculations. Especially, degenerate and nearly degenerate states omnipresent in polyatomc molecules lead to enhanced sensitivity to extremely weak perturbations, which may be utilized for the test of fundamental physical laws and the search for physics beyond the Standard Model, including the temporal and spatial variation of fundamental constants, parity violation, and time-reversal symmetry violation.^[3] 

Recently, our groups accomplished the first quantitative spin-vibronic analysis of the lowest electronic states of nonlinear MORs, including calcium methoxide (CaOCH₃),^[4] calcium ethoxide (CaOC₂H₅),^[5] and calcium isopropoxide [CaOCH(CH₃)₂].^[6] Experimentally, laser-induced fluorescence/dispersed fluorescence (LIF/DF) and cavity ring-down (CRD) spectra of the ^~A²E -^~X²A₁ electronic transition of CaOCH₃ (C_3v), the ^~A₁²A’’/ ^~A₂²A’-^~X²A’ transition of CaOC₂H₅ (C_s), and the ^~A₁²A’/ ^~A₂²A’’-^~X²A’ transition of CaOCH(CH₃)₂ (C_s) were recorded under jet-cooled conditions. An essentially constant ^~A₂ - ^~A₁ energy separation for different vibronic levels is observed in the LIF spectra of each radical, which is attributed to the spin-orbit (SO) interaction and, in the cases of CaOC₂H₅ and CaOCH(CH₃)₂, the non-relativistic effects as well. Electronic transition energies, vibrational frequencies, and spin-vibrational eigenfunctions calculated using the complete active space self-consistent field (CASSCF) and the coupled-cluster (CC) methods have been used to predict the vibronic energy level structure and simulate the recorded LIF/DF spectra. Although the vibrational frequencies and Franck-Condon (FC) factors calculated under the Born-Oppenheimer approximation and the harmonic oscillator approximation reproduce the dominant spectral features well, the inclusion of the Jahn-Teller (JT), the pseudo-Jahn-Teller (pJT), and the SO interactions, especially those between the ^~A₁/^~A₂ and the ^~B states, induces a number of off-diagonal FC matrix elements leading to additional vibronic transitions, which significantly improve the accuracy of the spectral simulations. A spin-vibronic Hamiltonian has been developed in a quasi-diabatic basis for the spectral simulation. CRD spectra were used for the accurate determination of the FC constants and vibrational branching ratios (VBRs), which are directly related to future laser cooling schemes and efficiencies.

To support future laser-cooling experiments and the quest for new physics, we have also developed a new spectroscopic model to simulate and fit the spin-ro-vibronic structure in high-resolution electronic spectra of molecules in nearly degenerate states,^[7] e.g., the ^~A₁ and ^~A₂ states of CaOR (C_s or C₁) free radicals. The sophisticated coupling scheme of angular momenta in low-symmetry free radicals has been modeled and quantified. In this talk, I will try to prove, using real-world examples, that rotational and fine-structure analysis of spectra involving nearly degenerate states can aid in vibronic analysis and interpretation of effective molecular constants.