Tuesday, January 24, 2012
Chemical Physics Seminar
Dynamics of Liquids and Interfaces Studied with Ultrafast 2D IR Vibrational Echo Spectroscopy
Michael D. Fayer, David Mulvane Ehrsam and Edward Curtis Franklin Professor of Chemistry, Department of Chemistry, Stanford University
Catalysts are important throughout chemistry. Heterogeneous catalysts, in which a surface is functionalized with the catalyst, are particularly useful. A heterogeneous catalyst makes it a different phase than the reactants in solution, simplifying separation and preventing leaching of the catalyst into the reaction mixture. Until now, it has not been possible to study the structural and environmental dynamics of heterogeneous catalysts, and in general the dynamics of molecules at surfaces and interfaces. Here ultrafast 2D IR vibrational echo experiments on a submonolayer of a heterogeneous catalyst are presented. To introduce the 2D IR vibrational echo technique, first two other problems are discussed. Vibrational echo chemical exchange spectroscopy is used to directly measure the dissociation and formation of organic solute-solvent complexes in solution. Then experiments that determine the hydrogen bond dynamics of water are discussed. The water experiments use spectral diffusion as the observable. Then the 2D IR vibrational echo experiments that measure spectral diffusion of phenanthroline Rhenium tricarbonyl chloride ((phen)Re(CO)3Cl) bound with a linker to an SiO2 surface are presented. The very small number of molecules in a submonolayer presents a severe experimental challenge. The heterogeneous photocatalyst (phen)Re(CO)3Cl reduces CO2 to CO. (phen)Re(CO)3Cl bound to the SiO2 solvent in air (dry, no solvent) shows structural dynamics on the 150 ps time scale. Thus, even in the absence of solvent, there are significant rapid molecular motions of the catalytic monolayer. When the identical surface sample is immerse in chloroform (wet), the dynamics speed up to 50 ps. These values are compared to (phen)Re(CO)3Cl as a homogeneous catalyst dissolved in chloroform where the dynamics occur on the 5 ps time scale. To extract the structural spectral diffusion dynamics, they must be separated from vibrational excitation transport induced spectral diffusion. The nature of the excitation transport problem is described. These studies are the first 2D IR vibrational echo experiments of any surface or interfacial molecular system. They open a new approach for understanding the nature of and processes involving surface bound and interfacial molecular systems.