Localized Detection of Inelastic Electron Tunneling Using a Scanning Tunneling Microscopy

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This grant from the Experimental Physical Chemistry Program supports research of J. Baldeschwieler and W.Kaiser of California Institute of Technology and the Jet Propulsion Laboratories respectively. The award funds research aimed toward the realization of examining the atomic motions inside individual molecules on surfaces. It is proposed to demonstrate the applicability of the scanning tunneling microscope (STM) to the detection of localized inelastic electron tunneling. Simple systems which entail the essential physics for inelastic electron tunneling spectroscopy of adsorbates will be studied. They are carefully chosen to be stable, easily prepared, and amenable to isotopic substitution to facilitate unambiguous assignment of spectral features to inelastic tunneling via excitation of adsorbate vibrational modes. A liquid-helium immersed scanning tunneling microscope designed specifically for the proposed work will be constructed; it will be a modification of the STM design used at the Jet Propulsion Laboratory of the California Institute of Technology for superconductor phonon spectroscopy, the most sensitive low-temperature spectroscopy measurement published to date. Initial efforts will be directed toward detecting Si-H and Si-D vibrational stretching modes on hydrogenated and deuterated degenerate silicon surfaces. Similar measurements are planned for hydrogen (or deuterium) rich small molecules chemisorbed to gold and silicon surfaces. These simple systems can be used to optimize experimental and instrumental parameters for inelastic tunneling spectroscopy with the STM. The project will establish lower limits for the detectability of inelastic electron tunneling and may demonstrate the applicability of the STM to the detection of inelastic electron tunneling due to excitation of adsorbate vibrational modes. The proposed investigation constitutes a step toward the realization of spatially resolved vibrational spectroscopy of individual adsorbed molecules together with three dimensional imaging of their binding environments. This is a potential application of the STM which would be of profound scientific and technological importance.