Our experimental research program is aimed at the investigation of molecules adsorbed on surfaces using the Scanning Tunneling Microscope (STM) to study the structure of such species. This particular line of investigation has enormous potential for future applications of STM to chemical problems. We have successfully imaged a number of surface adsorbates, including synthetic polypeptides and long-chain, functionalized hydrocarbons. Of particular interest are molecules containing chemical functional groups that exhibit unusual contrast (either high or low tunneling current) when imaged with the STM. Such groups serve as STM markers or “chromophores” that allow us to unravel the structures of surface films. Marker functionalities such as S atoms, Br atoms, and -COOH have been employed to study the chirality of molecules adsorbed at the interface between a racemic mixture and a solid surface. Separate domains can be observed on the solid surface corresponding to left- and right-handed forms of a chiral molecule. We are also using STM to probe chemical reactions of small organic halides on iron oxide surfaces in ultrahigh vacuum. Ultimately, these studies will provide insight into the reaction mechanisms for organic pollutants adsorbed on subsurface soil particles, which typically have a significant iron oxide content. The STM is a powerful tool for unraveling the structure of solid surfaces, as well as for studying monolayer films at liquid-solid and vacuum-solid interfaces, and we expect it to provide much information about this important “two-dimensional” state of matter.
A second part of our research program is aimed at the study of chemical dynamics. We investigate molecular collisions that lead either to chemical reaction or to the exchange of energy between molecules. In particular we have developed the infrared diode laser absorption probe technique to investigate collisions between molecules. Most recently this technique has been used to study the relaxation of molecules with “chemically significant” amounts of energy (E=50100 Kcal/mole). The energy transfer mechanism for these high energy species is one of the key steps in the Lindemann unimolecular reaction scheme, which has been studied with only limited success for nearly seventy years. The current experiments in our laboratory are aimed at studying high energy (“hot”) molecules that are literally ready to explode into small molecular and atomic fragments. We interrupt this explosion process by colliding an inert molecule with the exploding one, thereby substantially reducing the hot molecule¹s energy. By studying the final vibrational and rotational states and the translational energy of the inert molecule, we are able to establish the quantumstatescattering picture for this process with undreamed of accuracy. The probability of producing a given quantum state as a result of a collision or chemical reaction is a sensitive function of the shape of the transition state and the potential energy surface which govern molecular interactions. Quantum-stateresolved studies of collisions and chemical reactions are expected to lead to a fundamental understanding of the role of vibrational, rotational, and translational coordinates in reacting systems.
Kwang Taeg Rim, Thomas Müller, Jeffrey P. Fitts, Kaveh Adib, Nicholas Camillone, III, Richard M. Osgood, Enrique R. Batista, Richard A. Friesner, S. A. Joyce, and George W. Flynn, “An STM and Theoretical Study of Competitive Reactions in the Dissociative Chemisorption of CCl4 on Iron Oxide Surfaces”, J. Phys. Chem. B 108, 16753-16760 (2004)
Markus Lackinger, Thomas Müller, T.G. Gopakumar, Falk Müller, Michael Hietschold and George W. Flynn, “Tunneling Voltage Polarity Dependent Submolecular Contrast of Naphthalocyanine on Graphite – a STM Study of Close Packed Monolayers under UHV Conditions”, J. Phys. Chem. B, 108, 2279-2284 (2004)
Yanhu Wei, Kavita Kannappan, George W. Flynn, and Mathew B. Zimmt, “Scanning Tunneling Microscopy of C2h Symmetry Anthracene Derivatives on Graphite: Chain Length Effects on Monolayer Morphology”, J. Am. Chem. Soc., 126, 5318-5322 (2004)
Dalia G. Yablon, Deniz Ertas, Hongbin Fang, and George W. Flynn, “An STM Investigation of the Adsorption of Mixtures of Fatty Acids and Substituted Acids at the Solution-Graphite Interface”, Israel J. Chem. 43, 383-392 (2003)
Kwang Taeg Rim, Jeffrey P. Fitts, Thomas Müller, Kaveh Adib, Nicholas Camillone III, Richard M. Osgood, S. A. Joyce, and George W. Flynn, “CCl4 Chemistry on the Reduced Selvedge of a ?-Fe2O3(0001) Surface: A Scanning Tunneling Microscopy Study”, Surf. Sci. 541, 59-75 (2003)
Natalie Seiser, Kavita Kannappan, and George Flynn, “Long Range Collisional Energy Transfer from Highly Vibrationally Excited Pyrazine to CO Bath Molecules: Excitation of the v=1 CO Vibrational Level”, J. Phys. Chem. A 107, 8191-8197 (2003)
Thomas Müller, George Flynn, Anna T. Mathauser and Andrew Teplyakov, “Temperature Programmed Desorption Studies of n-Alkane Derivatives on Graphite: Desorption Energetics and the Influence of Functional Groups on Adsorbate Self-Assembly”, Langmuir, 19, 2812-2821 (2003)
K. Adib, G. G. Totir, J. P Fitts, K. T. Rim, T. Müller, G. W. Flynn, S. A. Joyce, and R. M. Osgood, “Chemistry of CCl4 on Fe3O4(111)-2×2 Surfaces in the Presence of Adsorbed D2O Studied by Temperature Programmed Desorption”, Surf. Sci. 537, 191-204 (2003)
K. Adib, N. Camillone III, J. P. Fitts, K. T. Rim, G. W. Flynn,, S. A. Joyce, and R. M. Osgood, “Dissociative Adsorption of CCl4 on the Fe3O4(111)-2×2 Selvedge of ?-Fe2O3(0001)”, Surf. Sci. 524, 113-128 (2003)
Thomas Müller, Dalia G. Yablon, Ryan Karchner, David Knapp, Mark H. Kleinman, Hongbin Fang, Christopher J. Durning, Donald A. Tomalia, Nicholas J. Turro, and George W. Flynn, “AFM Studies of High Generation PAMAM Dendrimers at the Liquid/Solid Interface”, Langmuir 18, 7452 (2002)