D. Heskett

The interaction range in alkali metal-promoted systems

Abstract The available experimental data of systems in which a molecule, mainly CO, is coadsorbed with an alkali metal on a metal surface are summarized with regard to the range of the alkali-coadsorbate interaction. We find that the primary promotion interaction is short-range and localized to 1–2 CO molecules per alkali adatom and is independent of initial alkali coverage. In addition, we find evidence, based primarily on vibrational data, for a longer-range alkali-molecule interaction as well, which is substantially weaker than the short-range interaction.

The coadsorption of oxygen and potassium on Ru(001): Evidence for the formation of K–O compounds

In order to study an adsorption system where bond formation between coadsorbates competes with the adsorbate–metal substrate bond, we have investigated the interaction between oxygen and a potassium monolayer on Ru(001). At low exposures of oxygen (0.4 L), vibrational, photoemission, Auger, and workfunction data indicate the formation of a KO2 species. EELS spectra are characterized by an intense K–O stretch at 240 cm−1 and the absence of Ru–O stretching modes. Auger spectra indicate a K:O stoichiometry of 1:2 and photoemission spectra contain features indicative of O–O bond formation. Subsequent exposure of oxygen results in the adsorption of atomic oxygen with a characteristic Ru–O stretching vibration at 615 cm−1. Annealing of the K–O layer results in the decomposition of KO2 to Kad+Oad, both being bonded more strongly to the metal substrate than the individually adsorbed species and thus indicating through‐metal interactions between atomic oxygen and potassium.

A correlation between anomalous electronic and vibrational properties of Chemisorbed molecules

Abstract The photoelectron spectra of weakly chemisorbed CO and N 2 molecules exhibit a multitude of satellite lines due to multi-electron excitations, i.e., the simple single particle picture of photoionization does not apply to these systems. In all chemisorption systems for which data are presently available, the observation of multi-electron excitations in the valence spectrum is always accompanied by an anomalous behavior for the energy of the molecular vibration as a function of coverage. The energy of such a vibrational mode decreases or remains constant with increasing coverage in these weak chemisorption systems, in contrast to what is seen in “ordinary adsorbates” or predicted by theory for adsorbate dipole-dipole coupling. The correlation between the vibrational spectral data and photoemission data is so consistent that there would appear to be a common explanation for these electronic and vibrational properties. We offer a possible explanation based on: (a) recent ab initio theoretical calculations, which incorporate important electronic correlation effects, and (b) a model previously proposed by Pritchard for the infrared data. The proposed model consists of the following main features. In weakly chemisorbed diatomic systems the 2π orbital (unoccupied in the free molecule) is not involved in the bonding to the substrate in the neutral system. The screening of the hole produced by photoionization is then a non-adiabatic process in which the unoccupied 2π-like orbital (slightly above the Fermi energy) of the neutral ground state falls below the Fermi level and becomes occupied in the ion state. This leads to strong satellite lines in the valence spectra. If the 2π orbital in adsorbed CO or N 2 is not occupied in the ground state, then the 5σ orbital is primarily responsible for the bond to the substrate. Since this orbital is slightly antibonding with respect to the two atoms of the diatomic, the molecular stretching frequency can decrease as the adsorbate coverage increases and the adsorbate-substrate bond weakens (Pritchards model).

Nitrogen-induced reconstruction of Cu(110): Formation of a surface nitride

Abstract Using the techniques of LEED, TDS and HREELS, we have studied the adsorption of atomic nitrogen on Cu(110). We find that the nitrogen overlayer exhibits a very sharp and stable (2×3) LEED pattern and unusual thermal desorption behavior. We interpret these results as indicating a nitrogen-induced reconstruction of the Cu(110) surface. Our HREELS measurements allow us to assign the binding sites of the nitrogen atoms to the long bridge sites on the surface. Similarities to bulk copper nitride, Cu 3 N, suggest that the adsorption and surface reconstruction of N/Cu(110) can be regarded as the formation of a surface copper nitride.

Oxygen and pyridine on Ag(110) studied by second harmonic generation: Coexistence of two phases within monolayer pyridine coverage

Abstract The adsorption of oxygen and pyridine on Ag(110) in UHV is studied by second harmonic generation (SHG) from the surface. The adsorption of oxygen at room temperature (atomic) and at 110 K (molecular) causes a monotonic decrease in the second harmonic intensity with coverage which obeys simple Langmuir kinetics. Model fits to the data yield initial sticking coefficients, s0, of 0.0049 and 0.0088 for atomic and molecular oxygen adsorption, respectively. In contrast to oxygen adsorption, the changes in SHG signal induced by the exposure of Ag(110) to pyridine are more complicated and cannot be fit to a single-site Langmuir adsorption model. The SHG measurements indicate the presence of two distinct phases in the pyridine monolayer. A model, which depicts a broad transition from a flat-lying pyridine configuration at low coverages to a stand-up configuration at higher coverages, reproduced both the SHG and the work function measurements. The flat-lying phase dominates at low coverage, but the stand-up phase prevails when the coverage exceeds 0.3 monolayers. s0 for pyridine adsorption at 110 K was determined to be close to unity.

Molecule-molecule and molecule-metal interactions of CO/Ru(001) and N2/Ru(001) studied by angle-resolved photoemission

Abstract Angle-resolved photoemission with a synchrotron source has been used to study in detail the valence electronic properties of CO and N 2 chemisorbed on the Ru(001) surface. CO/Ru(001) was investigated in two distinct coverage regimes corresponding to ( 3 × 3 ) R 30° and (2 3 × 2 3 ) R 30° ordered phases. N 2 was studied in the ( 3 × 3 ) R 30° phase. Binding energies of the CO valence levels are in good agreement with previously published results. The shape resonance positions of the 5σ and 4σ levels were measured and were found at ∼ 28 and 37 eV, respectively, and are discussed in detail in comparison with results of other systems. The CO bandwidths were measured and correlate well with measurements on other systems. The bandwidth of the N 2 5σ valence level was found to be substantially smaller than that of CO. An explanation is offered in terms of multi-electronic excitations associated with the N 2 levels. In all cases, our results are consistent with a perpendicular orientation of the CO and N 2 molecules.

Coverage dependent phase transition of pyridine on Ag(110) observed by second harmonic generation

Second harmonic generation from a Ag(110) surface is shown to be sensitive to submonolayer converages of O2, O, pyridine, and benzene. The second harmonic intensity as a function of exposure indicates that the orientation and bonding of pyridine adsorbed at 110 K changes at about 0.3 L.

Non-metallic behavior of cesium on GaAs(100)

Abstract The Cs/GaAs(110) interface provides a model system for investigations into the initial stages of metallization of a semiconductor surface. We have examined the valence states of both clean GaAs(100) and Cs/GaAs(110) by angle-resolved photoemission spectroscopy. For Cs exposures at room temperature, the Cs coverage does not exceed one monolayer and the resulting interface is non-metallic. The Cs coverage can be increased by cooling the substrate to ∼190 K, whereupon the second Cs layer is metallic.

The first interlayer spacing of Ta(100) determined by photoelectron diffraction

A photoelectron diffraction study of Ta(100), employing the surface shifted 4 f core levels, has been performed to determine its first interplanar spacing. Photoemission intensities of the bulk and surface shifted 4 f core levels were measured as a function of photon energy at normal emission and near the 〈111〉 symmetry direction of the crystal. The binding energy of 21.7 eV (23.6 eV) for the bulk 4 f 7/2 (5/2) core level, and the large surface core level shift of 0.75 eV, result in well‐resolved peaks in the photoemission spectra with areas that are easily measured independently. We find that the ratio of the surface to bulk core level intensities at normal emission exhibits large oscillations as a function of photon energy due to diffraction effects. Intensity modulation is also seen in the off‐normal intensities, but the magnitude is much smaller. We have performed a multiple‐scattering photoemission calculation for several first interlayer spacings ranging from the bulk value to a 15% contraction. The...

Electron loss spectra from thin alkali films on Al(111)

Angle resolved inelastic electron scattering has been used to measure the energy and dispersion of the electronic excitations for Na, K, and Cs thin films on Al(111) as a function of thickness. The energy loss spectrum for a two layer film is dominated by a collective excitation similar to that seen for a thick film. This ‘‘surface plasmon’’ mode for a two‐layer film exhibits a negative dispersion even larger than that observed for the thick film, even though the damping is appreciably larger for the thin film. The loss spectra in the submonolayer region are fundamentally different, apparently dominated by single particle excitations. The transition from a single particle to a collective excitation occurs quite abruptly as the second layer begins to form.