Molecular Vibrational Energy with High Time and Space Resolution

University Of Illinois At Urbana-Champaign, Urbana IL

Investigators

Abstract

TECHNICAL SUMMARY 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.