George W. Flynn
A few years ago, we began an entirely new experimental research program aimed at the investigation of molecules adsorbed on surfaces by 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 the long chain sulfur containing molecules that are physisorbed at the liquid solid interface of a graphite surface. In all compounds of this type that we have studied so far, the sulfurs appear as unusually bright spots in the STM images, suggesting an enhanced probability for tunneling over these atoms and their possible use as STM chromophores.
Marker groups 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. The STM is a powerful tool for unraveling the structure of solid surfaces as well as for studying monolayer films at liquid/solid interfaces, and we expect it to provide many insights into this important, two dimensional state of matter!
Over the past year, we have brought on-line a variable temperature UHV (ultra-high vacuum) STM. Our current UHV-STM efforts focus on studies of iron oxide surfaces as part of our involvement in EMSI (Environmental Molecular Sciences Institute).
Another 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=50-100 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 70 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 molecule's energy.
By studying the final vibrational and rotational states and the translational energy of the inert molecule, we are able to establish the quantum state scattering 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.
In principle quantum state resolved studies of collisions and chemical reactions will lead to a fundamental understanding of the role of vibrational, rotational, and translational coordinates in reacting systems.
S.R. Qiu, H.-F. Lai, M. J. Roberson, M. L. Hunt, C. Amrhein, L.C. Giancarlo, G. W. Flynn, and J. A. Yarmoff, "A Surface Chemistry Investigation of the Removal of Contaminants in Aqueous Solution by Iron", Langmuir, 16, 2230-2236 (2000) [view article - pdf]
E.T. Sevy, C. A. Michaels, H. C. Tapalian, and G.W. Flynn, "Competition Between Photochemistry and Energy Transfer in UV-excited Diazabenzenes: II. Identifying the Dominant Energy Donor for Supercollisions", J. Chem. Phys., 112, 5844-51 (2000)
E.T. Sevy, M.A. Muyskens, S.M. Rubin, and G.W. Flynn, "Competition Between Photochemistry and Energy Transfer in UV-excited Diazabenzenes: I. Photofragmentation Studies of Pyrazine at 248 nm and 266 nm", J. Chem. Phys., 112, 5829-43 (2000)
Leanna C. Giancarlo, Hongbin Fang, Luis Avila, Leonard W. Fine, and George W. Flynn, "Molecular Photography in the Undergraduate Laboratory: Identification of Functional Groups Using Scanning Tunneling Microscopy", J. Chem. Ed., 77, 66-71 (2000)
H. Fang, L.C. Giancarlo, and G. W. Flynn," Identification of the Conformation of Individual Molecules by Scanning Tunneling Microscopy", J. Phys. Chem. B, 103, 5712-5715 (1999)