In this award, funded by the Chemical Structure, Dynamics and Mechanisms Program of the Chemistry Division, Prof. Timothy C. Steimle of Arizona State University and his postdoctoral fellows, graduate, and undergraduate research students will study the high-resolution optical and microwave spectroscopy of metal-containing small molecules. A systematic study of metal mono- and dihydrides (MH, MH2), monofluorides (MF) and hydroxides (MOH) for the early transition metal group (scandium to copper) will be performed, with particular emphasis on the determination of electric and magnetic dipole moments, and magnetic hyperfine interactions. This study will reveal insights into the nature of chemical bonding in metal containing molecules, and allow for an assessment of the reliability of computational methodologies. Metal containing molecules such as the di- and tri-atomic systems targeted in this research represent model for understanding a variety of important reactions involving metals, including catalytic reactions. A second effort will focus on high atomic number diatomic metal halides and oxides which are the focus of parity non-conservation investigations. Parity non-conservation is a topic that concerns the fundamental symmetry properties of the universe and the effects are greatly enhanced in polar high atomic number metal containing diatomic molecules.

In addition to the aforementioned broader scientific applications of the proposed research to cutting-edge experiments in exciting areas of modern physics research, Prof. Steimle maintains a small but diverse research group, with whom he works closely to train in modern instrumental methods and sophisticated quantum mechanics. The group benefits from a number of collaborations -- both foreign as well as domestic -- which provides his group members with valuable interdisciplinary experience that crosses international borders.

Project Report

A major goal of modern experimental physical chemistry is to establish a molecular level understands of chemical reactions. A secondary goal is to use molecules to explore new hypotheses of the nature of the universe. Molecules are most readily probed using spectroscopy (i.e. the interaction of light and matter). In this project we have utilized the power of modern laser-based spectroscopy to probe the character of small gas-phase metal containing diatomic and triatomic molecules. The use of gas-phase samples, as opposed to the more readily available solid phase sample, assures that the phenomena probed are intrinsic to the individual molecules and not due to the interaction between these molecules. To probe the bonding of transition metal containing molecules, which is relevant to a molecular level understanding of catalysis, we have recorded and analyzed the optical spectra of the simplest molecules: scandium hydride, ScH, titanium hydride, TiH. Scandium and titanium are the first two transition metal elements. A major objective of this project was to provide highly quantitative information about the properties of these molecules which can be used to assess the ability of various computational methodologies being developed to predict the chemistry of metals. The theoretical formulism for predicting this chemistry relies upon computing a multi-dimensional, potential energy surface (PES) (i.e. the interaction energy as the atoms and electrons move away from the most stable arrangement). Careful evaluation of the performance of the various computational methodologies being developed for PES predictions can be achieved via a comparison of predicted and experimentally measured properties. The small molecules that we study are the species associated with the bound portions of the multi-dimensional PES. Emphasis has been placed upon the experimental determination of the permanent electric dipole moment, μe, which is routinely predicted. To a first approximation, me for a diatomic molecule is a measure of the polarity of the chemical bond. In addition to being essential for assessing the computational methodologies, the interaction of μe with applied electric filed is used to manipulation of kinetic energy of a molecule (e.g. slowing molecules to produce ultra-cold samples). We demonstrated that the commonly used computational methodology of density functional theory (DFT) is unable to accurately predict μe even for these very simple systems. The transition metal containing molecules thorium monoxide, ThO, tungsten monocarbide, WC, and ytterbium monofluoride, YbF, were also investigated using ultra-high resolution laser-based spectroscopy. These molecules are heavy (i.e. large atomic number) and polar (i.e. large me ) and are being used to determine the magnitude of the of electric dipole moment of the electron (eEDM), de. Contrary to the customary chemist view of the electron as a point charge, this fundamental particle, which is a lepton, has a very small charge distribution. The fact that leptons have a charge distribution implies a violation of both mirror symmetry and time-reversal invariance. It was realized nearly 40 years that instead of directly measuring de using a free electron, a significant enhancement in sensitivity could be achieved by indirectly measuring de when the electron is bound to a heavy molecules. The enchantment in sensitivity is due to very high velocities, which approach the speed of light, of the electrons in the region of the heavy nuclei. At these velocities relativistic effects become important. Basically, the molecule is functioning as a very inexpensive, but highly efficient, particle (electron) accelerator. In order for molecular-based de measurements to come to fruition, we have experimentally measured the properties (bond lengths, me , electronic states, etc.) of ThO, WC, and YbF. We have also investigated the bonding in silicon timers, Si3. Si3 plays an important, yet poorly understood, role in chemical vapor deposition process. It might be expected that silicon, Si, and carbon, C, would have very similar chemical behavior because they are in the same column in the periodic table of the elements. Surprisingly, the chemistry of the two elements is radically different. For example, clusters of carbon atoms have a propensity of forming linear molecules whereas clusters of silicon atoms tend to form cyclic molecules. This can, in the most general sense, be attributed to the more "metallic" nature of Si compared to C. It was observed that Si3 exhibits a strong interaction between the vibrational motion (i.e. the stretching and twisting of the structure away from that of an equilateral triangle) and the electronic motion. Given that the making and breaking of bonds is dictated by the electronic motion, this coupling is relevant to Si-chemistry. In this work we: a) identified and assigned strong visible electronic absorptions of gas-phase Si3; b) determined ground and excited state vibrational frequencies; c) developed a more effective method for describing the vibrational/electronic coupling. We have also developed a new sensitive spectroscopic method for remote monitoring of Si3 in energetic environments (e.g. plasmas and flames).

Agency
National Science Foundation (NSF)
Institute
Division of Chemistry (CHE)
Type
Standard Grant (Standard)
Application #
1011996
Program Officer
Colby A. Foss
Project Start
Project End
Budget Start
2010-09-01
Budget End
2013-08-31
Support Year
Fiscal Year
2010
Total Cost
$438,678
Indirect Cost
Name
Arizona State University
Department
Type
DUNS #
City
Tempe
State
AZ
Country
United States
Zip Code
85281