B. Reihl

Controlled cluster condensation into preformed nanometer-sized pits

We have performed scanning tunneling microscopy (STM) in ultrahigh vacuum and transmission electron microscopy (TEM) on silver and gold clusters grown in preformed nanometer-sized pits on the surface of highly oriented pyrolytic graphite. We describe the preparation method and evaluate the three-dimensional shape of the clusters using a combination of STM and TEM applied to the same cluster sample. The nanometer-sized pits were essential to fix the clusters in position when using STM. The influence of the tip shape on the STM imaging of nanometer-sized clusters is discussed.

Modification of the Shockley-type surface state on Ag(111) by an adsorbed xenon layer

Abstract The Shockley-type surface state for the clean and a Xe covered Ag(1xa01xa01) surface is studied with scanning tunneling microscopy and scanning tunneling spectroscopy at a temperature of 5 K. The minimum of the parabolic dispersion shifts from −67 meV below E F =0 for the clean Ag(1xa01xa01) surface to +52 meV with one layer of Xe adsorbed, hence the surface state becomes completely unoccupied after Xe adsorption. The dispersion is determined by measuring energy-dependent, locally-resolved, tunneling spectroscopy maps. These show standing wave patterns as local density of states formed by electron scattering at surface steps, which reveal equal effective masses with and without the Xe layer adsorbed. A line width analysis of the energetic onset of the surface state gives a slightly smaller lifetime of the surface state after Xe adsorption, in sharp contrast to the results for image states. We discuss the results and conclude that the wavefunction of the Shockley-type surface state stays close to the metal surface also after Xe adsorption, and that the observed shifts cannot be explained by the simultaneous change of the workfunction, but will be given mainly by the changes of the surface potential on an atomic scale.

Electronic structure and magnetism of actinide compounds: 6d versus 5f electrons (invited)

We have studied the electronic structure of cleaved (100) single crystals of UAsxSe1−x, UxTh1−xSb, UxY1−xSb, and USbxTe1−x with high resolution (ΔE<150 meV) photoemission using synchrotron radiation 10⩽hν⩽130 eV. In contrast to the general belief we find significant differences in the photoemission density of states near EF for these compounds. We conclude that these differences are mainly due to valence d electrons and that U has a quasi‐localized 5f3 configuration in resonance with itinerant 6d states. This explains the quite similar magnetic behavior of UxTh1−xSb and USbxTe1−x, both of which change from highly anisotropic 〈100〉 antiferromagnets (x≳0.85) to 〈111〉 ferromagnets (x≲0.85), i.e., with an increasing number of 6d electrons. Namely, the smaller ordered moment in UTe and the gradually decreasing moment in UxTh1−xSb for 1.0⩾x⩾0.3 are attributed to the 6d moments which are polarized antiparallel to the 5f moments.

The electronic structure of Ag(110)c(4 × 4)C60 and Au(110)(6 × 5)C60

Abstract C 60 chemisorbs and forms ordered overlayers on both Ag(110) and Au(110)2 × 1: Ag(110)c(4 × 4)C 60 and Au(110)(6 × 5)C 60 . We use direct (UPS) and inverse photoemission spectroscopy (IPS) to examine the occupied and unoccupied electronic structure of Ag(110)c(4 × 4)C 60 and IPS to examine the unoccupied electronic structure of Au(110)(6 × 5)C 60 . Comparing the data with multilayer spectra we observe clear and unequivocable signatures of final and initial state effects in the spectra. The final state effects take the form of screening of the ionic final state. Charge transfer from Ag(110) into first layer C 60 molecules is directly observed by UPS and the interaction of first layer C 60 molecules with the substrate is further evidenced by a broadening of C 60 features, ascribed to a splitting of molecular orbitals. The work functions of Ag(110)c(4 × 4)C 60 and Au(110)(6 × 5)C 60 are found to be equal to that of bulk C 60 . The combination of the work function measurements and the spectroscopy give new insights into the bonding of C 60 on these surfaces.

Photon emission from adsorbed C60 molecules with sub-nanometer lateral resolution

We present the first experimental demonstration of spatially resolved photon emission of individual molecules on a surface. A scanning tunneling microscope (STM) was used as a local electron source to excite photon emission from hexagonal arrays of C60 molecules on Au(110) surfaces. Specifically, we show that in maps of photon emission intensities, C60 fullerenes appear as arrays of individual light emitters 4 Å in diameter and separated by 10 Å. Comparison with simultaneously recorded STM images reveals, that most intense emission is detected when the STM tip is centered above a molecule. The results demonstrate the highest spatial resolution of light emission to date using a scanning probe technique.

On the optical properties of coldly deposited silver films: Comparison of eels and reflectivity spectra

Abstract Electron energy loss (EEL) spectra of coldly deposited silver films show the same surface plasmons as observed for Ag(111) and room temperature deposited films. No EEL structure corresponds to the strong anomalous optical absorption centeree at 2.4 eV. We conclude that the latter must be caused by excitations within the porous film.

Tunneling spectroscopy on silver clusters at T=5 K: size dependence and spatial energy shifts

Abstract We have studied silver clusters grown in nanopits on a graphite surface using low-temperature scanning tunneling spectroscopy (STS) at T =5xa0K. The pronounced peak structure measured in the STS data of the clusters is interpreted to have its origin in the quantized electronic structure of the cluster–surface system. Additionally, a systematic spatial energy shift was observed for some spectral features in d I /d V maps on top of the clusters.

The electronic structure of alkali-metal layers on semiconductor surfaces

Alkali-metal layers on semiconductor surfaces are model systems for metal-semiconductor contacts, Schottky barriers, and metallization processes. The strong decrease of the work function as a function of alkali-metal coverage is also technically made use of. Recently, however, interest in these systems is growing owing to ongoing controversial discussions about questions like: Is the adsorbate system at monolayer coverage metallic or semiconducting, and does the metallization take place in the alkali overlayer or in the top layer of the semiconductor? Is the bonding ionic or covalent? What ist the absolute coverage at saturation? What are the adsorption sites? Do all alkali metals behave similar on the same semiconductor surface? We try to answer some of the questions for Li, Na, K and Cs on Si(111)(2×1), K and Cs on Si(111)(7×7) and on GaAs(110), and Na and K on Si(100)(2×1) employing the techniques of direct and inverse photoemission.

Internal structure of C60 fullerence molecules as revealed by low-temperature STM

We have performed Scanning Tunneling Microscopy (STM) in ultra-high vacuum at low temperatures (5 and 50K) of unordered and ordered C60 layers adsorbed on a Au(110) surface. STM topographs of the frozen C60 molecules reveal four symmetric patterns within single molecules, which may be associated with different orientations of the fullerenes on a highly corrugated gold substrate.

π* and σ* molecular orbitals of condensed films of chlorobenzenes and hexafluorobenzene observed by inverse photoemission

The electron affinity levels of condensed films of benzene, chlorobenzene, 1.4‐dichlorobenzene, s‐trichlorobenzene, and hexafluorobenzene on Cu(111) have been determined by isochromat inverse photoemission spectroscopy. In addition to the π* orbitals of the benzene ring, low‐lying σ* orbitals are observed in the chlorine and fluorine‐substituted molecules. Comparison to electron transmission spectra allows the assignment of the unoccupied molecular orbitals and the determination of the relaxation energies in going from the gas phase to the solid state. These relaxation energies are found to decrease in the series of benzene to trichlorobenzene.