INELASTIC ELECTRON-TUNNELING - AN ALTERNATIVE MOLECULAR-SPECTROSCOPY

The basic theoretical and experimental concepts required for an unders tanding of inelastic electron tunneling spectroscopy (IETS) are presen ted. While most of the applications of IETS to date have centered on s urface chemical analysis, the thrust of the present review is to prese nt IETS as an alternative molecular spectroscopy. Comparison of IETS, IR, and Raman data obtained in the vibrational region of the spectrum and of IETS and absorption and reflectance data taken in the electroni c region of the spectrum will be made. The difference in selection rul es in IETS and in optical spectroscopy is emphasized. Numerous example s of optically forbidden transitions observed as strong bands in IETS are presented. Spin and orbitally forbidden electronic transitions in the IETS are often as strong as, or stronger than, their optically all owed counterparts. We will identify those spectral features that are u nique to the tunneling environment and cannot be associated with norma l molecular spectra. Finally, we will give a brief introduction to som e new innovations that may make tunneling spectroscopy a more useful t echnique for the nonspecialist.