Tunneling spectroscopy performed with the scanning tunneling microscope is used to study image-type surface states. The tunneling tip causes a Stark shift and expansion of the hydrogenic image-state spectrum, permitting a clear resolution of the individual states. A simple theoretical model provides a quantitative connection between the tunneling data and both previous and new inverse-photoemission data.Image states are an interesting set of surface states, which have attracted a great deal of attention in recent years. 1 " 6 These hydrogenlike states are bound to a surface by the response of the substrate to the presence of the electron, and kept outside the surface by the reflective properties of the substrate. A theoretical model in which electronic motion along and perpendicular to the surface is taken to be independent yields the following relation for the binding energy of each member of the hydrogenic series:where n is the principal quantum number, e n is the purely hydrogenic component of the binding energy, k ( | is the wave vector parallel to the surface, m* is the effective mass, and E ncorT is the shift of the bottom of the band caused by deviations from perfectly freeelectron motion along the surface. These states have been previously observed by LEED 2 and by /c-resolved inverse-photoemission spectroscopy (KRIPES) . 3 " 7 Also, the important role played by the image potential in the vacuum tunnel current has recently been pointed out. 7 For image states, the electric field characterizing a tunnel junction has the effect of causing a Stark shift in the states of the hydrogenic spectrum. The evolu-tion of the spectrum due first to corrugation effects and then to the electric field is shown in Fig. 1. The Stark shift has the effect of continuously shifting and expanding the image-state spectrum, with its accumulation of states at the vacuum level E VSiC , into the geometric resonance spectrum associated with the Vshaped potential created by the substrate and the field. Such geometric resonances were considered by Jason 8 to explain oscillations in field ionization, and by Gundlach, 9 who predicted oscillating tunneling I-V characteristics, which were observed experimentally in semiconductor planar tunnel junctions 10 and metalmetal interfaces 11 as well as on Au tips in vacuum tunneling. 12 The tunneling measurements we describe add the scanning capability of the scanning tunneling microscope (STM) 1314 to the well-established capabilities of traditional tunneling spectroscopy. The benefits of this addition are numerious, e.g., bare surfaces become accessible to the full arsenal of UHV surface treatments and analysis, and it becomes possible to select and probe a suitable small portion of the surface. Figure 2 illustrates representative tunneling spectra for several surfaces. They were taken with a voltage modulation of A V = 0.2 V at v = 200 to 400 Hz, a frequency above the response of the feedback system Ni (100) i i > O) H>--te ILU 1Z Mxi \ (a) n = co 10 ^ n=2 yn = 1 TUNNEL _ 1D BARRIER T 'P -10HFIG...

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The unoccupied π* bands of epitaxial overlayers of benzene, naphthalene, anthracene, tetracene, perylene, and coronene on a Ag(111) surface have been studied by angle resolved inverse photoemission spectroscopy. A comparison with HAM/3 MO calculations and electron transmission gas phase measurements enables the assignment of the π* affinity levels of these organic systems. In conjunction with previous angle resolved photoemission, optical absorption, and near edge x-ray absorption studies a complete picture of their valence band structure and morphology evolves. Optical and x-ray absorption excitation energies are discussed in the light of the experimentally determined one-electron correlation diagram. Gas phase–solid relaxation energies of the affinity levels are found to decrease in the series of benzene to tetracene (1.7→0.5 eV) attributed to the increasing size of the involved molecular orbitals. For benzene a low lying σ*-shape resonance has been identified in the isochromat inverse photoemission spectrum.

A phase transition has been observed in the system pyridine adsorbed on Ag(lll) at 100 K by near-edge x-ray-absorption fine-structure measurements in real time. At low pyridine coverages an angle between the ring plane and the surface plane of 45° ±5° was observed. This phase converts sharply at a submonolayer coverage to a phase with an angle between the ring plane and the surface plane of 70° ± 5°. Continued exposure gradually leads to a randomly oriented multilayer.PACS numbers: 68.35.Rh, 68.35.Bs, 78.70.Drn There is widespread interest in the pyridine-silver system because of its importance for surface-enhanced Raman scattering. 1 " 3 In particular, some proposed enhancement models involve charge-transfer excitations from the metal to affinity levels of the adsorbate, 4,5 which are also probed by the near-edge x-rayabsorption fine-structure (NEXAFS) technique. 6 " 8 Pyridine (C5H5N) is electronically characterized by a nitrogen lone-pair orbital. Pyridine chemisorption on metal surfaces may therefore have model character for determining the relative contributions of lone-pair and TT bonding to the chemisorption bond. Here we report the coverage-dependent molecular orientation of pyridine chemisorbed on Ag(lll) at 100 K determined by NEXAFS.Demuth, Christmann, and Sanda 9 studied the chemisorption of pyridine on clean Ag(lll) surfaces at --140 K with vibrational electron-energy-loss (VEELS) and uv-photoemission spectroscopies (UPS). They observed a phase transition at about half a monolayer coverage from a nearly flat-lying 7r-bonded pyridine phase to an inclined N-bonded phase. Similar coverage-dependent orientational phase transitions have been obtained by VEELS for pyridine on Ni(001) 10 and Pt(llO). 11 Orientational phase transitions also occur as a function of temperature. This has been shown for pyridine on Ni(001) by VEELS 10 and for pyridine on Pt(lll) in a recent NEXAFS study. 12 The NEXAFS measurements on Pt(lll) indicated a low-temperature pyridine state with an apparent angle between the ring plane and the surface plane of 52° which converts at 7^300 K to a high-temperature state with a corresponding tilt angle of 74°. This result, as well as UPS and electronic EELS data for pyridine on Ag(lll), 1314 casts some doubt on the existence of flat-lying pyridine molecules on well-defined surfaces at low coverages.NEXAFS studies on molecules which are only weak-ly perturbed by chemisorption are particularly useful in determining the orientation of the molecules relative to the surface. 6 " 8 For low-symmetry surfaces this includes the determination of the azimuthal orientation. 15 The polarization dependence of NEXAFS transitions is due to the validity of dipole selection rules for photoabsorption. The analysis of NEXAFS is especially unambiguous when TT resonances occur resulting from transitions of a Is electron into unfilled antibonding TT states. These TT resonances are rather sharp compared with a-shape resonances so that background subtraction is straightforward. With high-brightness storage ring...

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Very high-efficiency green organic light-emitting devices based on electrophosphorescence Applied Physics Letters 75, 4 (1999);

Near edge X-ray absorption fine structure (NEXAFS) has been used to establish the orientation of the six-membered ring molecules benzene, pyridine, pyrazine and s-triazine adsorbed on Cu and Ag single crystal surfaces. For submonolayer coverages of pyridine on Ag(111) an orientational phase transition has been observed with NEXAFS in real time

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