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

路易斯维尔大学（UofL）的刘金军教授和他的研究小组在化学学部化学结构、动力学和机制A （CSDM-A）项目资助下，利用先进的激光技术研究分子中原子核和电子的复杂运动，包括电子围绕原子核的“轨道运动”、电子的自旋、原子核的振动和旋转运动。在分子中，这些运动是“量子化的”，这意味着它们不是连续的，而是离散的。因此，分子可以处于具有不同能量和“量子态”的分离“能级”上。理解能级结构以及能级之间的能量分布和转移对物理科学的许多学科都是至关重要的。然而，实验得到的分子能级结构数据往往极其复杂，难以理解，因为电子和原子核的不同运动是重合的，相互耦合的。此外，许多量子态是“暗”的，也就是说，它们不能用单个激光束从分子的最低能量状态进入。这两个因素常常妨碍对分子能级结构的全面和定量的理解。UofL团队计划通过开发新的光谱技术来克服这两个障碍，这种技术使用不是一个，而是两个激光束来分析分子。使用两个激光束提供了简化实验数据所必需的能级选择性。假设中间量子态被明智地选择，它也可以创造通往暗态的“弯路”。在实验中，激光被困在两个反射镜之间，形成光学腔，这大大增加了激光与分子相互作用的时间，从而提高了探测灵敏度。UofL团队首先研究的分子是二氧化氮（NO2）和硝酸盐自由基（NO3）。这两种分子在大气化学中都起着重要的作用。NO2是光化学大气污染的主要污染物之一，而NO3是夜间对流层的主要氧化剂。UofL团队还与计算化学家合作，设计他们的实验并解释预期的实验数据。该项目重点发展两种高分辨率、高灵敏度的光谱技术：双共振腔衰荡（DR-CRD）和激发发射泵浦腔衰荡（SEP-CRU）。这两种方法都是基于高灵敏度的CRD技术。它们具有与双光子光谱相关的优势，包括简化光谱，亚多普勒线宽，以及访问分子暗态的能力，由于禁止选择规则，小跃迁偶极矩或不利的弗兰克-康登因子，单光子光谱技术无法访问分子暗态。利用这两种新的激光光谱技术，研究了振动（振动-电子）耦合（如Jahn-Teller效应和伪Jahn-Teller效应）和相关的分子内相互作用（如自旋轨道相互作用）的定量信息。第一个目标分子NO2和NO3是研究振动相互作用的两个典型分子。用DR-CRD和SEP-CRU技术在室温和喷射冷却条件下（T~ 1k）对它们进行了研究。根据计算化学和光谱模型预测的两步激发的中间态，UofL团队研究了这两种自由基的基态和低处激发态的自旋-旋转-振动-电子能级结构。本文将两种已有的光谱模型（振动分析模型和旋转分析模型）结合起来，模拟和拟合了实验所得光谱中的自旋-旋转振动结构，揭示了振动相互作用的复杂机理。就更广泛的科学影响而言，本项目开发的实验技术和理论模型是研究多势能面（PESs）光谱和动力学的独特有效工具。参与该项目的博士后和博士生在前沿激光光谱学技术、分子物理、理论化学和实验数据的计算机模拟等方面积累了宝贵的经验。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.