This project is an experimental study of vibrational energy in condensed-phase molecules using ultrafast laser spectroscopy. The focus is on understanding how vibrational energy moves over molecular dimensions from one location to another. This fundamental knowledge is needed to better understand chemical reactivity and to understand heat dissipation in molecular machines. Two techniques have already been developed in the Dlott laboratory that allow experimenters to input vibrational energy at one location of a molecule and probe its arrival at one or more other locations. In the IR-Raman technique that will be used to study molecular liquids such as a series of substituted benzenes, the energy is input with a tunable IR pulse and detected by a time series of anti-Stokes Raman spectra. In the ultrafast flash-thermal conductance technique that will be used to study molecular monolayers adsorbed on metal substrates, heat is input by flash-heating the metal layer and probing the molecular adsorbate with coherent vibrational sum-frequency generation (SFG) spectroscopy. The monolayer method is especially useful for studying molecular machinery, but SFG provides only an overall measure of heat flow, as opposed to anti-Stokes Raman that reveals which vibrational states carry the energy. Recent advances in surface-enhanced Raman spectroscopy from the Dlott laboratory will improve the sensitivity of anti-Stokes Raman enough to probe molecular monolayers. Combined with SFG, these experiments will reveal both the rate of heat flow and the mechanism of heat flow through a series of crafted molecular structures.
NON-TECHNICAL SUMMARY
All machinery generates heat. When the machine is the size of a molecule, the familiar concepts of heat transport no longer apply. This project seeks to understand the fundamental science of heat transport through molecules using advanced laser technology that produces light pulses less than one trillionth of a second in duration. With these advanced lasers, researchers in the Dlott group at the University of Illinois can input heat (vibrational energy) into one part of a molecule and measure how long it takes the heat to reach other parts of the molecule located a few angstroms (1 angstrom = 10-10 meters is about the diameter of one of the molecule's atoms) away. In this project, Dlott group researchers will develop new techniques to improve the measurement of vibrational energy, and will study how systematic changes of the molecular structure can speed up or slow down the heat transport. Ultimately this work will lead to a fundamental understanding of heat at the molecular level and provide the underlying knowledge needed to engineer molecules for specific heat transport applications, enabling new technologies to help the US remain economically competitive. The work will be performed by graduate students and postdoctoral researchers at the University of Illinois, who will learn to design, construct and operate advanced laser systems for studies of molecular machinery as they progress in their training to become world-class scientists. The focus on heat flow processes, which are familiar to laypersons as well as all scientists, helps insure the wide dissemination of the results of our work to technical journals and popular science media.
All machines generate heat. When the machine is the size of a molecule, the familiar concepts of heat transport no longer apply. Instead mechanical energy in molecules appears as quantum mechanical vibrational excitations. This project seeks to understand the fundamental science of vibrational energy transport through molecules using advanced laser technology that produces light pulses less than one trillionth of a second in duration. With these advanced lasers, researchers in the Dlott group at the University of Illinois can input heat (vibrational energy) into one part of a molecule and measure how long it takes the heat to reach other parts of the molecule located a few angstroms (1 angstrom = 10-10 meters is about the diameter of one of the molecule's atoms) away. In this project, Dlott group researchers developed new techniques to improve the measurement of vibrational energy, and studied how systematic changes of the molecular structure can speed up or slow down the heat transport. Ultimately this work will lead to a fundamental understanding of heat at the molecular level, and provide the underlying knowledge needed to engineer molecules for specific heat transport applications, enabling new technologies to help the US remain economically competitive. The work was performed by graduate students and postdoctoral researchers at the University of Illinois, who learned to design, construct and operate advanced laser systems for studies of molecular machinery as they progressed in their training to become world-class scientists. Two methods were used that were unique in their ability to probe point-to-point vibrational energy transfer. The first, called three dimensional IR-Raman spectroscopy, studies vibrational energy in liquids. The three dimensions are the energies of the initial vibrations produced with a short laser pulse, the energies of the vibrations to which the initial energy was transferred, and the time interval. The second looks at single layers (monolayers) of molecules attached to metal surfaces. A laser pulse is used to flash heat the metal surface and heat then flows from the surface to the molecules via the molecule-surface attachment point. Then other laser pulses measure how long it takes the energy to reach other locations of the molecules. These techniques are quite different in details but similar in concept. The difference between the two techniques is that the first looks at molecules randomly floating around in a liquid. The information from this type of measurement is needed to understand how molecules in a liquid undergo chemical reactions. The second looks at molecules bonded to and arranged on a surface. The information from this type of measurement is needed to understand how to construct electronic devices where each component is a molecule. Some of the most significant results were: Discovery of molecules that behave as vibrational energy diodes, permitting one-way flow of vibrational energy. Real time measurements of vibrational in long-chain molecules that can be used as molecular wires. We measured the speed of energy flow along the chains and the rate of the undesirable chain kinking process that happens when the molecules get too hot. Students and postdocs working on these projects learned how to develop novel laser technologies and they learned about the latest advances in vibrational spectroscopy. They received advanced training used to understand how excited molecules interact with their environment. They also learned how to synthesize and fabricate advanced nanomaterials. The results of these studies were published in scientific journals and in book chapters. Also these results were disseminated by oral or poster presentation at national meetings and by oral presentations to small groups, in workshops, and to chemistry and materials science departments at other universities. Outreach activities performed during this grant period included: Popular lectures on shock compression science at the Pinhead Institute in Telluride, at a local elementary school and at the "science cafe" in a local pub. A girl's summer science day camp. Scientific experiments for the Illinois State Science Olympics.
