This dissertation discusses the theoretical basis and experimental applications to improving the capability of the STM in chemical and optical sensitivity. Traditional STM methods have achieved unprecedented spatial resolution, but suffer from a lack of sensitivity to chemical structure and composition. A new method of imaging, based on inelastic electron tunneling spectroscopy (IETS) measurement of hydrogen molecules is developed. The interaction of plasmon excitations to electronic states of a metal nano-cluster is also studied, allowing for better understanding of the mechanisms involved in the plasmon – electron coupling.

Since its application at the single molecule level in the STM was realized, IETS has been used to identify different molecules through their vibrational signal. In recent experiments, rotational excitation of H2 was detected on metal and insulator surfaces. It was found that the energy of these excitations depend sensitively on the local chemical environment. By monitoring the rotational and vibrational IETS signal of the H2 across the molecule, a more chemically sensitive image can be constructed. When the method is applied to imaging magnesium porphyrin (MgP) on Au (110), different components of the molecule can be observed at different energies. These differences are indication of how the various components interact with the H2.

Optical sensitivity of the STM manifests in the detection of photons emitted from the tunnel junction. Previous experiments have shown that we can map the excitation of molecular fluorescence with sub-Angstrom resolution. For applications in photochemistry and catalysis, understanding how plasmons interact with photons and electrons is crucial. Light emission from Au nanoclusters on oxide shows strong correlation with their electronic states. The interaction between plasmon mode in the junction and electronic states of the nano-clusters is further studied through clusters of different sizes and dimers. Emission of light from molecular orbitals is also investigated in panhematin and azulene molecules. Coupling of molecular orbital and vibronic states to junction plasmon is found and visualized through light emission spectrum and imaging.