Identification of individual surface adsorbates

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We have successfully demonstrated that individual adsorbates on solid surfaces can be identified with scanning tunneling microscope (STM) light emission spectroscopy. STM has proven to be a very powerful tool for visualizing surface nano-structures with atomic spatial resolution. While an atom or a molecule adsorbed on a surface can be visualized by STM, their species cannot be identified from the STM image alone. Methods for identifying individual adsorbates are highly desirable in many fields of surface science and nanotechnology. When electrons are injected into a sample surface from the tip of a STM, visible light is emitted. Since the cross-section of the electron beam emitted by the STM tip is on the order of the atomic size, the tunneling electrons are selectively injected into the area with the corresponding size. Hence when the tip is located over an adsorbate imaged by the conventional STM operation, the excited optical spectrum (i.e., STM light emission spectrum) will reflect its properties. The purpose of the present project was to demonstrate that surface adsorbates can be identified by STM light emission spectra. For this purpose we have measured the STM light emission spectra of various combinations of adsorbates and substrates ; oxygen atoms on Cu(110), Ni(110) and Ag(110), hydrogen atoms on Ni(110), CO molecules on Cu(110), Pt(110), and Ni(110), and Rhodamine 6G molecules on highly ordered pyrolytic graphite (HOPG). We found that the optical spectra of these adsorbates are different from those of the corresponding substrates. This result clearly indicates that identification of surface adsorbates becomes possible by STM light emission measurements.