Research
A revolution is underway as scientists learn to harness technologies based on quantum mechanics to solve important societal problems, ranging from renewable energy to biological science and computation. One way chemists can advance these efforts is by studying metal-ligand bonds that are important in such diverse areas as catalysis, astrochemistry, environmental remediation, and quantum computation. In the Augenbraun Lab, we pursue two interrelated goals: (1) to elucidate fundamental aspects of molecular bonding and (2) to discover technologically-useful molecules that can be controlled by laser light. Some representative molecules of interest include molecules with "multiple optical cycling centers," like CaCCSr, and coinage-metal carbides, like AuC. We use colorful lasers to study how these molecules absorb and emit light, and then unravel the patterns observed to characterize molecular electronic, vibrational, and rotational structures. This is an exciting journey: from synthesizing exotic molecules to gaining a full understanding of how these molecules bond and react!
Scientific Goals
A cartoon depiction of photon cycling. The molecule CaSH can be excited from its ground electronic state to a low-lying electronic state by driving a primarily non-bonding orbital that has little effect on the vibrational motion of the molecule. The Ca atom in this case acts as an optical cycling center (an "OCC").
Molecules for Quantum Science
Some of the most promising platforms for quantum computing and quantum sensing rely on the ability to control the motion and quantum states of atoms or small molecules. A critical tool is optical cycling, a process in which atoms/molecules rapidly and repeatedly absorb photons and then decay back to their initial energy levels. With optical cycling, scientists can use light to control the movement of molecules and to probe the fragile quantum mechanical effects that form the heart of modern quantum information processing.
Within the past decade, researchers have just begun to identify polyatomic molecules that contain "optical cycling centers" (OCCs). One of our lab's central goals is to identify new polyatomic molecules that can support optical cycling and laser cooling, including molecules containing multiple optical cycling centers such as CaCCYb.
We expect these molecules to exhibit fascinating quantum phenomena, like entanglement and super-/sub-radiance. In addition, by placing an OCC at one end of a molecule and a deformed nucleus (like Ta or W) at the other, we may be able to sympathetically cool complex nuclei with outstanding sensitivity to fundamental symmetry violations.
As an example of these ideas, this figure shows the computed electronic orbitals for a series of alkaline-earth pseudohalides (CaOH, CaCH3, CaNH2, and CaSH), representing a systematic symmetry descent.
Gas-Phase Organometallic Chemistry
Despite the common perception that coinage metals (copper, silver, and gold) are relatively inert species, recent research has shown that they can act as highly useful catalysts. In realistic catalysts, these metals often appear as part of large coordination complexes that makes understanding the intermediates and mechanisms difficult. Our goal in this project is to study the Cu-C, Ag-C, and Au-C bonds using individual, isolated molecules so that we can learn about nature's simplest coinage metal-carbon bonds. We will study species like diatomic CuC, polyatomic AuCCH, AgCC, and so on. A primary interest is to explore the role of relativistic effects in gold catalysis of organic reactions. This work involves both experimental identification of coinage-metal carbides using laser spectroscopy, and theoretical modeling of their electronic structure. Many of these molecules have never been produced in the laboratory before--so Williams students will be the first to observe their spectra!
Research Opportunities for Students
There are many different ways that students can get involved in answering these research questions: from building instrumentation to analyzing data, simulating molecular spectra, performing "first principles" quantum chemical calculations, and preparing reagents that are used in gas-phase reactions. Students in this lab will get to see how exciting it is to be involved in all different aspects of an experiment and its associated theoretical interpretation.
Some projects that current/future students will pursue are listed below. If any of these opportunities seem appealing please reach out to find out more!
Molecular Beam Source
To perform spectroscopy, we must first produce the molecules we plan to study. Because many of the molecules of interest to our group are highly reactive radicals, they must be produced in a vacuum chamber. Students will build a versatile molecular beam source that allows us to produce metal-containing molecules at temperatures just a few degrees above absolute zero. Not only does working at low temperatures simplify the molecular spectra, it also allows us to study the fascinating world of cold chemistry (where the laws of quantum mechancs, rather than classical kinetics, govern the outcome of reactions)! The image above shows a cryogenic beam source that is capable of producing molecular beams around 2 K.
Laser Assembly
In addition to producing the molecules we want to study, we need some way to excite them - in our group, this is done using a combination of pulsed and continuous-wave lasers operating in the visible part of the electromagnetic spectrum (roughly between 400 and 700 nm). On this project, students will be involved in the setup and control of laser systems: we will make sure we can produce high-power, stable, and tunable light that is capable of exciting molecular electronic transitions. We will also build a set of optical detectors that are sensitive to the light emitted by molecules as they decay back to ground electronic levels. The image above shows a continuous-wave dye laser operating around 630 nm.
Quantum Chemistry
Understanding of the molecular spectra that we record will be complemented by ab initio quantum chemical computations. Students will learn how to run state-of-the-art quantum chemistry programs to predict the properties of molecules, including the electronic and vibrational structure, molecular geometry, dipole moments, etc. Shown above is a set of molecular orbitals computed using density functional theory in the ORCA software package. The image above shows a relatively deeply-bound orbital in an alkaline-earth propanoate radical.