High-resolution spectroscopy of weakly chemisorbed species using a low-temperature scanning tunneling microscope (STM): CO/Cu(111)

High-resolution spectroscopy with an STM (STS) concerning the electronic aspects of adsorption is presented. Comparing spectra taken directly at an adsorbate and in its vicinity with microscopically identical tips allows to separate tip-induced features from such originating from the chemical interaction of the adsorbate with the substrate. The effects of tip contamination are studied by comparing spectra taken with bare and CO-functionalized tips. It is found that STS can have resolution power not only in the direct vicinity of the Fermi edge but also a few eV into the occupied and unoccupied states. We present exemplary measurements on CO/Cu(111) where as a new feature an unoccupied, probably 5σ-derived state was found.