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Inelastic Electron Tunneling Spectroscop~ or lETS, provides a unique technique for electronically monitoring the vibrational modes of molecul (;5 adsorbed on a metal oxide surface. Since the discovery of the phenomena by JAKLEVIC and LM1BE in 1966, lETS has been developed by a number of scientists as a method for studying the surface chemistry of molecular species adsorbed on aluminum oxide. Recent applications of lETS include investigations of physical and chemical adsorption of hydrocarbons, studies of catalysis by metal particles, detection and identification of trace substances in air and water, and studies of biological molecules and electron damage to such molecules. lETS has been employed to investigate adhesive materials, and studies are currently in prog ress to investigate corrosion species and corrosion inhibitors on aluminum and its alloys. Electronic transitions of molecules have also been studied by lETS. The recent development of the "external doping" technique, whereby molecu lar species can be introduced into fabricated tunnel junctions, opens the door for a vast new array of surface chemical studies by lETS. lETS is rap idly becoming an important tool for the study of surface and interface phe nomena. In addition to its role in surface studies, inelastic tunneling has proved extremely valuable for the study of the electronic properties of thin metallic films, and the recent discovery of light emission from inelastic tunneling promises to be of some importance in the area of device physics.
The power of rotational spectroscopy has long been demonstrated in the frequency domain by microwave spectroscopy, but its application in real space has been limited. Using a scanning tunneling microscope (STM) and inelastic electron tunneling spectroscopy (IETS), we were able to conduct real-space measurements of rotational transitions of gaseous hydrogen molecules physisorbed on surfaces at 10 K. The j=0 to j=2 rotational transition for para-H 2 and HD were observed by STM-IETS. It is also found that the rotational energy is very sensitive to its local environment, we could precisely investigate how the environmental coupling modifies the structure, including the bond length, of a single molecule with sub-Angstrom resolution. Due to this high sensitivity, the spatial variation in the potential energy surface can be quantified by the rotational and vibrational energies of the trapped H 2. The ability of the tip to drag along a hydrogen molecule as it scans over another adsorbed molecule combined with the sensitivity of the hydrogen rotational excitation recorded by IETS to its immediate environment lead to the implementation of rotational spectromicroscopy. Hydrogen rotational spectroscopy and microscopy provides novels approach toward visualizing and quantifying the intermolecular interaction as well as the intermediate processes of chemical reactions.
The observation of the vibrational spectra of adsorbed species provides one of the most incisive methods for und erst an ding chemical and physical phenomena on surfaces. At the present time, many approaches may be applied to studies of molecular vibrations on surfaces. Some of these are used on high-area solids of technological importance (e.g., heterogeneous catalysts) while others are applied to single-crystal substrates to gain better understanding under conditions of controlled surface structure. This book has attempted to bring together in one place a discussion of the major methods used to measure vibrational spectra of surface species. The emphasis is on basic concepts and experimental methods rather than a current survey of the extensive literature in this field. Two introductory chapters describe the basic theoretical aspects of vibrational spectroscopy on surfaces, dealing with normal modes and excitation mechanisms in vibrational spectroscopy. The remaining seven chapters deal with various methods employed to observe surface vibra tions. These are arranged in an order that first treats the use of various methods on surfaces that are not of the single-crystal type. It is in this area that the field first got started in the late 1940s with pioneering work by Terenin and others in the Soviet Union, and by Eisehens and others in the United States in the 1950s. The last four chapters deal with relatively recent methods that permit vibrational studies to be made on single crystal substrates.
With a 600mk homebuilt UHV STM system, we studied molecular vibration at the solid surface with inelastic electron tunneling spectroscopy (IETS) of Acetylene single molecules adsorbed on Cu(100) surface and revealed five new vibrational modes that were previously inaccessible to STM-IETS at 8K temperature. The identification of vibrational IETS features with normalized conductance change (Î4Ï3/Ï3) as low as 0.24% was demonstrated. Facilitated by the high energy resolution, we also revealed the anisotropic vibrational energy of carbon0́3monoxide (CO) molecule induced by substrate surface symmetry. The discrepancy in vibrational energy as small as 0.8meV can be resolved by STM-IETS. Our results also showed that the change in vibrational behavior of CO can be used to understand its environment. CO can be vertically transferred from substrate surface to STM tip and creates a scanning probe which has the characteristic vibrational signal of CO contained in the inelastic component of the tunneling current; i.e. the inelastic tunneling probe (itProbe). The itProbe senses the local potential energy landscape by imaging the spatial variations of CO hindered-translational mode and resolves the skeletal structure and bonding details of a surface adsorbed Cobalt-Phthalocyanine molecule (CoPc). The image contrast of itProbe also reveals the interaction between one CoPc and the substrate surface as well as the interaction paths between neighboring CoPcs.
This book has been compiled to give specialists, in areas that could be helped by tunneling spectroscopy, a rounded and relatively painless intro duction to the field. Why relatively painless? Because this book is filled with figures-A quick glance through these figures can give one a good idea of the types of systems that can be studied and the quality of results that can be obtained. To date, it has been somewhat difficult to learn about tunneling spectroscopy, as papers in this field have appeared in a diversity of scientific journals: for example. The Journal of Adhesion, J(}urnal (}f Catalysis, Surface and Interface Analysis, Science, Journal of the American Chemical Society, Physical Review-over 45 different ones in all, plus numerous conference proceedings. This diversity is, however, undoubtedly healthy. It indicates that the findings of tunneling spectroscopy are of interest and potential benefit to a wide audience. This book can help people who have seen a few papers or heard a talk on tunneling spectroscopy and want to learn more about what it can do for their field. Tunneling spectroscopy is presently in a transitional state. Its experi mental methods and theoretical basis have been reasonably well developed. Its continued vitality will depend on the success of its applications. Crucial to that success, as pointed out by Ward Plummer, is the adoption of tunneling spectroscopy by specialists in the areas of application.
Inelastic Electron Tunneling Spectroscopy (IETS) with the Scanning Tunneling Microscope (STM) is a novel vibrational spectroscopy technique that permits to characterize very subtle properties of molecules adsorbed on metallic surfaces. Its proposed symmetry-based propensity selection rules, however, fail to fully capture its exact mechanism and influencing factors; are not directly retraceable to an adsorbate property and are cumbersome. In this thesis, a theoretical approach was taken to improve them. An IETS simulation protocol has been developed, parameterized and benchmarked, and consequently used to calculate IETS spectra for a set of systematically related small molecules on copper surfaces. Extending IETS principles were deduced that refer to the tunneling state's vacuum extension, the selective activating/quenching of certain types of modes due to the moieties' electronic properties, and the applicability of a sum rule of IETS signals. Also, fingerprinting IETS-signals that enable discrimination between adsorbate orientations, the chemical nature of atoms and structural isomers were determined and a strategy using straightforward electronic density distribution properties of the isolated molecule to predict IETS activity without (large) computational cost was developed. This expertise was used to rationalize and interpret experimentally measured IETS spectra for adsorbed metalloporphyrins and metallophthalocyanines, being the first IETS studies of this large size. This experimental approach permitted to determine the current limitations of IETS-simulations. The associated identification shortcomings were resolved by conducting complementary STM-image simulations.