Laser spectroscopy of metal and semimetal molecules

在这个由化学部门化学结构、动力学和机制A （CDSM-A）项目资助的项目中，犹他大学的Michael D. Morse教授使用复杂的激光技术，对钛、铁、钴和镍等过渡金属与碳、氧、硅和硫等主要族元素之间的化学键强度进行了高度精确的测量。诸如此类的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药。它们也被自然界用于生命所必需的酶的活性位点。更详细和精确地了解这些系统中的化学键对于它们的准确建模是很重要的，反过来，这对于充分理解这些催化过程是必不可少的。此外，Morse教授正在使用相同的激光技术来研究硼团簇的化学键和电子结构。正如碳团簇随着富勒烯和碳纳米管的发现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，这可能在未来的应用中变得重要。这项研究对不寻常的电子特性进行了比以前更详细的研究。这项工作最重要的广泛影响是对新一代物理化学家的启发和培养。这项研究在技术上和概念上都具有挑战性。从事这些项目的学生不断地被迫对他们的仪器进行故障排除，并培养强大的解决问题的能力，这些能力可转移到许多不同的科学努力领域。在对当地社区的拓展努力中，莫尔斯博士继续为那些对物理化学概念有挑战的高中先修课程（AP）化学教师每年举办两次研讨会。这些讲习班提高了教师的能力。对物理化学的兴趣和知识，使他们能够激励新一代的高中生。本研究采用共振双光子电离光谱探测TiC、FeO、CoC等分子的吸收光谱，并在密集的振动光谱中观察到一个尖锐的预解离阈值，以高精度测量这些分子的键解离能。预解离阈值很容易识别，因为当达到这个阈值时，分子分解的速度比它们电离的速度快，导致质谱中的信号丢失。对阳离子（如TiC+、TiSi+等）的键离解能进行类似的测量，使用冷冷离子光解光谱仪，其中大量选择阳离子，冷却到低温，辐照，并检测产生的碎片离子。当在最低分离碎片极限附近存在较大密度的振动态时，一个尖锐的碎片开始以高精度识别键能。硼团簇的研究使用了先前的工作，其中获得了质量选择的硼团簇阴离子的光电子能谱，作为定位中性物质中的电子跃迁的指导。然后以比光电子实验中获得的分辨率高50倍的分辨率记录这些数据，从而允许对振动结构进行分析。这项工作，特别是关于过渡金属键能的工作，通过为改进量子化学方法的发展提供准确的基准，具有更广泛的影响，这些方法可能随后被用于成功地处理对于这种高精度实验工作来说太大的系统。这项工作的一个更广泛的影响来自于精确测量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工作。它还通过提供一个高度精确的数据库来测试这些方法，从而有助于开发计算精确的方法。

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.