INELASTIC ELECTRON-TUNNELING - AN ALTERNATIVE MOLECULAR-SPECTROSCOPY

The basic theoretical and experimental concepts required for an understanding of inelastic electron tunneling spectroscopy (IETS) are presented. While most of the applications of IETS to date have centered on surface chemical analysis, the thrust of the present review is to present 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 electronic region of the spectrum will be made. The difference in selection rules in IETS and in optical spectroscopy is emphasized. Numerous examples 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 allowed counterparts. We will identify those spectral features that are unique to the tunneling environment and cannot be associated with normal molecular spectra. Finally, we will give a brief introduction to some new innovations that may make tunneling spectroscopy a more useful technique for the nonspecialist.