Laser spectroscopy of metal and semimetal molecules

University Of Utah, Salt Lake City UT

Investigators

Abstract

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.