List of Faculty
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Imaging Chemical Bond Directions in Adsorbed Molecules
A novel technique to measure the chemical bonding directions in adsorbed molecules on single crystal surfaces is being applied to study the details of chemical bonding at surfaces. The method depends upon generating a Coulomb explosion within a chemical bond by electronic excitation, ejecting an ion fragment. This method allows one to observe the thermally averaged directionality of some of the internal bonds within an adsorbed species and to witness the dynamical behavior of the bond, giving information about the structure and on-site motion of the molecule. The technique, called ESDIAD (Electron Stimulated Desorption Ion Angular Distribution) was co-invented by a team of three including myself at NBS in 1974. We currently measure the ion angular distribution and ion speed using time-of-flight methods. Recently the method has been used, along with STM, to determine the bonding site location and the detailed structure of organic molecules adsorbed on metal and semiconductor single crystals. These measurements also connect the H+ yields from particular C-H bonds to the transmission coefficient for electrons coming from the substrate through the molecule to neutralize a short-lived hole in the molecule.
Photochemistry on Semiconductor Surfaces
Semiconductor surfaces, such as TiO2, exhibit the ability to convert photon energy to chemical energy which activates adsorbed molecules. This occurs when electron-hole pairs are produced in the solid and subsequent charge transfer to adsorbed species takes place. TiO2 activated by this means is a widely-used photooxidation catalyst for environmental remediation. We have recently shown by quantitative studies that the involvement of hole-traps in the TiO2 profoundly influences the magnitude of the hole-induced photodesorption of adsorbed O2, and measurements of the hole-trap density have been made for the first time. An optical technique for monitoring the electron concentration in electron trap sites near the bottom of the conduction band has been developed, allowing infrared spectroscopy to witness both the surface photochemistry as well as the excited electron concentration in the semiconductor at the same time. The work connects fundamentally to environmental cleanup, to solar cells and to chemical detectors.
Adsorption on Carbon Single Wall Nanotubes
As a result of strong physical adsorption in the cylindrical internal cavities in nanotubes, novel adsorption phenomena are present on these unique surfaces. Infrared spectroscopy can discriminate internally-bound molecules from those more weakly bound on external nanotube surfaces. Thermal desorption also readily resolves adsorption from internal and external surfaces as well as measuring the adsorbate population on the various sites. Such studies contribute fundamentally to understanding adsorption on carbon surfaces since nanotubes display a finite set of adsorption sites and energies, compared to high area technical carbon surfaces which exhibit a continuum of site energies. Experiments are planned in which strong adsorption centers, placed inside nanotubes, will act to reduce the reversibility of adsorption, using the nanotube interior as an efficient conduit to a strong internal binding site.
Adsorption on Ionic Solid Surfaces
Ionic crystals, such as MgO, expose defective cationic sites having reduced coordination numbers to their neighbor anions. The chemistry of these low coordination number sites will be probed by studies of the IR spectrum of test molecules such as CO whose C-O vibrational frequency and binding energy is strongly dependent on the electric field sampled at the cation site. Experimental work, done in conjunction with theoretical studies headed by Professor Matt Neurock of Chemical Engineering, are expected to yield new insights into the adsorptive and reactive properties of highly defective nanocrystals of metal oxides, used as sorbents in environmental remediation as well as in heterogeneous catalysis.
Recent Publications
T. Zubkov, G. Morgan, Jr. and J. T. Yates, Jr., “Spectroscopic Detection of CO Dissociation on Defect Sites on Ru(109): Implications for Fischer-Tropsch Catalytic Chemistry,” Chem. Phys. Lett., 362, 181 (2002).
J. Lee, I. A. Balabin, D. N. Beratan, J.-G. Lee and J. T. Yates, Jr., “Charge Transfer through Chemisorbed Organic Molecules – Neutralization of Ionization Processes at Local Sites in the Molecule,” Chem. Phys. Lett. 412, 171 (2005).
T. L. Thompson and J. T. Yates, Jr., “Monitoring Hole Trapping in Photoexcited TiO2 (110) using a Surface Photoreaction,” J. Phys. Chem. B 109, 18230 (2005).
P. Kondratyuk, Y. Wang, J. K. Johnson and J. T. Yates, Jr., “Observation of a One-Dimensional Adsorption Site on Carbon Nanotubes: Adsorption of Alkanes of Different Molecular Lengths,” J. Phys. Chem. B 109, 20999 (2005).
P. Maksymovych, D. C. Sorescu, D. Dougherty and J. T. Yates, Jr., “Surface Bonding and Dynamical Behavior of the CH3SH Molecule on Au(111),” J. Phys. Chem. B 109, 22463 (2005).
P. Maksymovych and J. T. Yates, Jr., “Unexpected Spontaneous Formation of CO Clusters on the Au(111) Surface,” Chem. Phys. Lett. 421, 473 (20
P. Maksymovych and J. T. Yates, Jr., “Propagation of Conformation in the Surface-Aligned Dissociation of Single CH3SSCH3 Molecules on Au(111),” J. Am. Chem. Soc. 128, 10642 (2006).
O. Byl, J-C. Liu, Y. Wang, W-L. Yim, J. K. Johnson and J. T. Yates, Jr., “Unusual Hydrogen Bonding in Water-filled Carbon Nanotubes,” J. Am. Chem. Soc. 128, 12090 (2006).
P. Maksymovych, D. C. Sorescu and J. T. Yates, Jr., “Gold-Adatom-Mediated Bonding in Self-Assembled Short-Chain Alkanethiolate Species on the Au(111) Surface,” Phys. Rev. Lett. 97, 146103 (2006).
T. L. Thompson and J. T. Yates, Jr., “Surface Science Studies of the Photoactivation of TiO2-New Photochemical Processes,” Invited Review Article for Photochemistry and Photophysics on Surfaces, Chem. Rev. 106, 4428 (2006).