Laser spectroscopy of metal and semimetal molecules

在这项由化学系化学结构、动力学和机制A(CDSM-A)计划资助的项目中，犹他大学的Michael D.Morse教授使用先进的激光技术，对过渡金属(如钛、铁、钴和镍)与主要族元素(如碳、氧、硅和硫)之间的化学键强度进行了高精度的测量。这类过渡金属被广泛用于将石油转化为有用的化学品和合成新的药物。它们也被自然界用在生命所必需的酶的活性部位。更详细和准确地了解这类体系中的化学键对于它们的准确建模是重要的，而这反过来又是充分理解这些催化过程的关键。此外，莫尔斯教授正在使用同样的激光技术来研究硼原子团的化学键和电子结构。正如随着富勒烯和碳纳米管的发现，碳团簇在技术上变得重要一样，硼团簇显示出不同寻常的化学性质，这些性质可能会在未来的应用中变得重要。这项研究比以前更详细地研究了不寻常的电子性质。拟议工作最重要的更广泛的影响是激励和培训新一代物理化学家。这项研究在技术和概念上都具有挑战性。从事这些项目的学生不断地被迫排除他们的仪器故障，并发展强大的解决问题的技能，这些技能可以转移到许多不同的科学努力领域。作为对当地社区的外展努力，莫尔斯博士继续为受到物理化学概念挑战的高中先修课程(AP)化学教师举办一年两次的研讨会。这些工作坊提升了教师的素质吗？对物理化学的兴趣和知识，让他们能够激励新一代高中生。该研究使用共振双光子电离光谱探测TiC、FeO、CoC等分子的吸收光谱，并利用在密集的振动光谱中观察到的尖锐的预解离阈值来高精度地测量这些分子的键离解能。预解离阈值很容易确定，因为当达到它时，分子解体的速度比它们被电离的速度更快，导致质谱图中的信号损失。用低温离子光解离谱仪对TiC+、TiSi+等阳离子的键离解能进行了类似的测量。在低温离子光解离谱仪中，对阳离子进行质量选择，冷却到低温，照射，然后检测生成的碎片离子。当在最低分离碎片极限附近有较大密度的振动态时，急剧的碎裂开始将键能量识别为高精度。对硼团簇的研究使用了以前的工作，其中获得了大量选择的硼团簇阴离子的光电子能谱，作为定位中性物种中电子跃迁的指南。然后，以比光电子实验中获得的分辨率高约50倍的分辨率记录这些信号，从而可以分析振动结构。这项工作，特别是关于过渡金属键能的工作，通过为开发改进的量子化学方法提供准确的基准，然后可以用来成功地处理对于如此高精度的实验工作来说太大的系统，产生了更广泛的影响。这项工作的一个更广泛的影响来自于精确测量的键离解能的广泛适用性。这是扩展我们的化学知识和化学直觉的基础性工作。它还通过提供一个可以用来测试这些方法的高度准确的数据库，促进了计算准确方法的发展。

In this project funded by the Chemical Structure, Dynamics, and Mechanisms A (CDSM-A) program of the Chemistry Division, Professor Michael D. Morse of the University of Utah is using sophisticated laser techniques to provide highly precise measurements of the strength of the chemical bonds between transition metals such as titanium, iron, cobalt, and nickel and main group elements such as carbon, oxygen, silicon, and sulfur. Transition metals such as these are widely used to transform petroleum into useful chemicals and to synthesize new pharmaceuticals. They are also used by nature in the active sites of enzymes that are essential for life. A more detailed and precise understanding of the chemical bonding in such systems is important for their accurate modeling, which is, in turn, essential for a full understanding of these catalytic processes. In addition, Professor Morse is using the same laser techniques to investigate the chemical bonding and electronic structure of boron clusters. Just as carbon clusters have become technologically important with the discovery of the fullerenes and carbon nanotubes, boron clusters show unusual chemical properties that may become important in future applications. This research investigates unusual electronic properties in far greater detail than has been accomplished previously. The most important broader impact of the proposed work is the inspiration and training of a new generation of physical chemists. The research is technically and conceptually challenging. Students working on these projects are continually forced to troubleshoot their instruments and to develop strong problem-solving skills, which are transferable to many different arenas of scientific endeavor. In an outreach effort to the local community, Dr. Morse continues to give twice-yearly workshops for high school Advanced Placement (AP) chemistry teachers who are challenged by physical chemistry concepts. These workshops enhance the teachers? interest in and knowledge of physical chemistry, so that they can inspire a new generation of high school students. The research uses resonant two-photon ionization spectroscopy to probe the absorption spectrum of molecules such as TiC, FeO, CoC, etc., and uses the observation of a sharp predissociation threshold in a dense vibronic spectrum to measure the bond dissociation energies of these molecules to high precision. The predissociation threshold is easily identified because when it is reached, the molecules fall apart faster than they can be ionized, leading to a loss of signal in the mass spectrum. Analogous measurements of the bond dissociation energies of cations, such as TiC+, TiSi+, etc. are undertaken using a cryo-cooled ion photodissociation spectrometer, in which the cation is mass-selected, cooled to low temperatures, irradiated, and the resulting fragment ion is detected. When there is a large density of vibronic states in the vicinity of the lowest separated fragment limit, a sharp fragmentation onset identifies the bond energy to high precision. The studies of boron clusters use previous work in which photoelectron spectra were obtained of mass selected boron cluster anions as a guide to locate electronic transitions in the neutral species. These are then recorded at a resolution that is about 50 times better than that obtained in the photoelectron experiments, allowing the vibrational structure to be analyzed. This work, particularly the work on transition metal bond energies, has broader impacts by providing accurate benchmarks for the development of improved quantum chemical methods that may then be used to successfully treat systems that are too large for such high precision experimental work. A broader impact of the work comes in the broad applicability of the precisely measured bond dissociation energies. This is fundamental work that extends our chemical knowledge and chemical intuition. It also contributes to the development of computationally accurate methods by providing a highly accurate database by which these methods may be tested.