The application of simple selection rules to photoemission from clean and adsorbate covered metal single crystals

The selection rules governing the uv-photoemission process from clean and adsorbate covered surfaces, as a consequence of the symmetry properties of the system, are discussed. It is shown that the symmetries of the initial states giving rise to features in the angle-resolved photoemission spectra can be readily derived from photon polarization dependent measurements. Symmetry based selection rules are also applied to the problem of adsorbate molecule orientation for the case of adsorbate covered surfaces.     

Inverse photoemission from solids: Theoretical aspects and applications

The current status of inverse photoemission from clean crystals is reviewed with special emphasis on theoretical aspects and applications. The article focuses on general photoemission theory, the effective barrier potential at metal surfaces and the calculation of inverse photoemission spectra for both bulk and surface electronic states. In particular the information originating from inverse photoemission on Shockley and image-potential surface states, effective masses of mage-potential states and spin-split surface states at ferromagnets is discussed.     

Electronic structure of semiconductor surfaces

Our present understanding of the electronic structure of semiconductor surfaces is reviewed. It is shown that photoemission and inverse photoemission are ideal techniques for probing occupied and unoccupied electronic states, respectively. All quantum numbers of an electron can be determined, i.e., energy, momentum, spin and angular symmetries. For simple systems, such as clean ordered surfaces with a small unit cell it is possible to understand the electronic structure from first-principles calculations. For complex systems, such as encountered during oxidation and dry etching one is restricted to measuring the properties determined by short-range order. Core level spectroscopy with synchrotron radiation is able to determine the oxidation state and the local bonding of surface and interface atoms.     

Quasiparticle effects on tunneling currents: a study of C2H4 adsorbed on the Si(001)- (2 x 1) surface

We present a first-principles calculation of the quasiparticle electronic structure of ethylene adsorbed on the dimer reconstructed Si(001)-(2x1) surface. Within the GW approximation, the self-energy corrections for the adsorbate states are found to be about 1.5 eV larger than those for the states derived from bulk silicon. The calculated quasiparticle band structure is in excellent agreement with photoemission spectra. Finally, the effects of the quasiparticle corrections on the scanning tunneling microscope images of the adsorbed molecules are shown to be important as the lowering of the C2H4 energy levels within GW strongly reduces their tunneling probability.     

Correct application of Fresnel’s equations for intensity analysis of angle-resolved photoemission data

The electromagnetic field relevant for the excitation process in angle-resolved photoemission is studied. We show that Fresnel’s equations together with the known bulk dielectric constants can be used to calculate the complex vector potential at the metal surface. A model is developed which accounts correctly for the special experimental geometry with focused light. It is used to calculate the variation of photoemission intensity with changing light incidence angle and polarization. Experimental data for the photoemission intensity as a function of light incidence angle are presented for direct transitions out of bulk, surface and adsorbate states at a Cu(1 1 0) surface. The comparison to our model shows that the application of copper bulk optical constants is justified even when electronic states are localized to the topmost atomic layer.     

Unoccupied electronic states in adsorbate systems

Experimental work on unoccupied electronic states in adsorbate systems on metallic substrates is reviewed with emphasis on recent developments. The first part is devoted to molecular adsorbates. Weakly chemisorbed hydrocarbons are briefly discussed. An exhaustive inverse photoemission (IPE) study of the CO bond to the transition metals Ni, Pb, and Pt is presented. Adsorbed NO is taken as an example to demonstrate the persisting discrepancies in the interpretation of IPE spectra. Atomic adsorbates are discussed in the second part. The quantum well state model is applied to interpret the surface states in reconstructing and non-reconstructing adsorption systems of alkali metals and hydrogen. A recent controversy on the unoccupied electronic states of the Cu(110)/O p(2×1) surface is critically reviewed. The quantum well state model is then compared to tight binding and local-density-functional calculations of the unoccupied bands and the deficiencies of the various approaches are pointed out. Finally, the relation between the surface state model and more chemically oriented models of surface bonding is briefly discussed.     

Chemically modified semiconductor surfaces: 1,4-phenylenediamine on Si(100)

The formation of inorganic/organic hybrid systems in ultra-high-vacuum (UHV) is studied for the adsorption of 1,4-phenylenediamine (PDA) and aniline on Si(100)(2 × 1) single crystal surfaces. X-ray photoemission spectra (XPS) of PDA and aniline adsorbed on silicon are interpreted in terms of an adsorbate configuration involving two covalent bonds between one amino group per molecule and the dangling bonds of the clean silicon surface. Vibrational high resolution electron energy loss spectroscopy (HREELS) allows us to identify the remaining amino groups in the PDA-adsorbate. Ultraviolet photoemission spectroscopy (UPS) and electron energy loss (EELS) data in the valence band range indicate that the aromatic ring stays intact after the chemisorption of PDA. Changes in work function and energetic shifts of the electronic transitions characterize the charge transfer between adsorbate and substrate. The experimental results are finally discussed in the framework of semi-empirical cluster calculations using MNDO-PM3.     

Angle resolved photoemission from adsorbates: Theoretical considerations of polarization effects and symmetry

Angular distributions of photoemission from chemisorption induced states and LEED spectra are calculated for the p (2×2) and the c (2×2) oxygen superstructure on the (001) surface of nickel. The obtained dependence of the photoemission intensity of the polar angle, on the azimuth angle of emission and on the direction of the light beam is discussed using mainly symmetry arguments (selection rules). This kind of analysis is shown to be useful for identifying the orbital symmetry or the symmetry of the adsorbate position without carrying out complicated calculations.     

Direct measurement of core-level relaxation dynamics on a surface-adsorbate system

The coupling between electronic states in a surface-adsorbate system is fundamental to the understanding of many surface interactions. In this Letter, we present the first direct time-resolved observations of the lifetime of core-excited states of an atom adsorbed onto a surface. By comparing laser-assisted photoemission from a substrate with a delayed Auger decay process from an adsorbate, we measure the lifetime of the 4d(-1) core level of xenon on Pt(111) to be 7.1+/-1.1 fs. This result opens up time-domain measurements of surface dynamics where energy-resolved measurements may provide incomplete information.     

Electron states of ultrathin Pd and Cu films on (001)Ag

We present the results of a theoretical calculation of the electronic structure of a monolayer of Pd and Cu deposited on the (001) surface of Ag. The adsorbate atoms are placed in a central site with the same lateral periodicity as a parallel plane of the substrate. The method of calculation is a parametrized LCAO with hopping integrals determined through a fit to bulk data. Charge transfer effects are taken approximately into account by assuming the intraatomic parameters to vary as the valence orbital ionization potentials. Energy bands and local densities of states are given and the nature and location of the adsorbate states are discussed. The results are compared with the experimental data: good agreement is found for the Pd on Ag system. For Cu on Ag we find that our assumptions do not explain the photoemission data and suggest that some clustering of Cu atoms occurs during the deposition.     

Electronic properties of (Co, Ag) self-organized nano dots on Au(1 1 1) vicinal surfaces

Electronic properties of (Ag, Co) nanostructures grown on Au(1 1 1) vicinal surfaces have been studied by angle resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy and spectroscopy (STM/STS). The growth and self-assembling of Co and Ag nano dots on Au(7 8 8) surface are described. Co island growth leads to the formation of repulsive energy barriers for the surface state, and subsequently to the appearance of confined states in between each group of four Co dots. On the contrary, when Ag nano dots are grown, the potential barrier for the surface electrons is not enough to suppress their dispersive behavior. Nevertheless, inside Ag islands appear new quantized states whose energies can be tailored by varying the deposition rate of the adsorbate and/or the Miller index of the vicinal surfaces. In both systems, high homogeneity of the electronic properties is achieved over a macroscopic scale.     

Fermi surface and anisotropic spin-orbit coupling of Sb(111) studied by angle-resolved photoemission spectroscopy

High-resolution angle-resolved photoemission spectroscopy has been performed on Sb(111) to elucidate the origin of anomalous electronic properties in group-V semimetal surfaces. The surface was found to be metallic despite the semimetallic character of bulk. We clearly observed two surface-derived Fermi surfaces which are likely spin split, demonstrating that the spin-orbit interaction plays a dominant role in characterizing the surface electronic states of group-V semimetals. The universality or dissimilarity of the electronic structure in Bi and Sb is discussed in relation to the granular superconductivity, electron-phonon coupling, and surface charge or spin density wave.     

Electron states at Zn(0001) surface

The electronic structure of the Zn(0001) surface is analyzed through a realistic slab calculation with an accurate tight-binding sp-d model hamiltonian. The results are compared with a recent photoemission investigation of this surface. The comparison with noble and near-noble metal surfaces, with reference to the surface states in the s-p gap at the centre of the surface Brillouin zone, is also discussed.     

Angle resolved photoemission from adsorbates: Theoretical considerations of polarization effects and symmetry

Angular distributions of photoemission from chemisorption induced states and LEED spectra are calculated for the p(2 × 2) and the c(2 × 2) oxygen superstructure on the (001) surface of nickel. The obtained dependence of the photoemission intensity on the polar angle, on the azimuth angle of emission and on the direction of the light beam is discussed using mainly symmetry arguments (selection rules). This kind of analysis is shown to be useful for identifying the orbital symmetry or the symmetry of the adsorbate position without carrying out complicated calculations.     

One-dimensional metal structures at decorated steps

It is shown how wire structures a few nanometers wide can be fabricated by decorating step edges at vicinal surfaces. Their growth modes and electronic states are studied using Scanning Tunneling Microscopy (STM) and inverse photoemission. The observed growth modes are two-dimensional analogs of Stranski-Krastanov growth and layer-by-layer growth in three dimensions, e.g., for Cu on stepped Mo(1 1 0) and W(1 1 0), respectively. Contrast between different metals is achieved in STM pictures by resonant tunneling via surface states and image states, with the latter providing a map of the work function. The limit of single atomic rows decorating step edges is studied by inverse photoemission, and an energy shift of 0.4 eV is found for electronic states of step atoms. We expect stripe structures to become useful for the study of two- vs one-dimensional magnetism, for magnetoresistive films, and in the design of anisotropic materials.     

Spin-resolved inverse photoemission of ferromagnetic surfaces

Inverse photoemission (IPE) with spin-polarized electrons provides a way to measure separately the exchange-split majority and minority bands in ferromagnets above the Fermi level. Consequently spin-resolved IPE turns out to be an outstanding technique for obtaining information on surface magnetism: the magnetization of the topmost atomic layer may be investigated by measuring the exchange splitting of electronic states that are localized within the surface layer. Theoretical models of ferromagnetism may be tested by observing the temperature behavior of bands which contribute to the ferromagnetism of the material. The magnetic coupling of an adsorbate to the ferromagnetic substrate may be studied by probing adsorbate-induced bands. Results for the Ni(110) surface serve as an illustration to discuss the status of spin-resolved IPE and its application to the field of surface magnetism.     

Photoemission and energy loss spectroscopy on semiconductor surfaces

The application of multiple experimental techniques to silicon surface studies is discussed. Two specific cases are considered: the chemisorption of oxygen on silicon at coverages up to one monolayer and the intrinsic surface states on cleaved and on annealed Si (111) surfaces. It is shown that by combining electron energy loss spectroscopy and ultraviolet photoemission spectroscopy, one can obtain an approximate energy level model for both occupied and unoccupied states     

Surface states,local bonding,and surface reconstruction: Na on Cu(110)

Occupied and unoccupied surface states on Na/Cu(110) have been investigated by photoemission and inverse photoemission. By measuring surface state positions at the same adsorbate coverage on the reconstructed and the unreconstructed surface it is demonstrated that the surface states contribute to the total energy balance of the Na induced missing row reconstruction. During the onset of the reconstruction an up-shift of the surface states depopulates an initially occupied CuCu bonding surface state and consequently destabilizes the topmost Cu atoms. The surface state wavefunctions are qualitatively analyzed in a thight-binding LCAO picture.     

Tuning dimensionality by nanowire adsorption on layered materials

The dimensionality of electronic states determines a number of physical phenomena such as phase transitions, transport, or superconductivity. Employing scanning tunneling microscopy combined with angle-resolved photoemission spectroscopy we demonstrate how the dimensionality of electronic states can be continuously tuned from three to two dimensions. This is achieved by adsorption of nanowires on surfaces of layered crystals without changing the chemical composition of the material. Exemplary results for Rb nanowires on TiTe2 are discussed with the help of electronic structure calculations.     

Electronic properties of hydrogen- and oxygen-terminateddiamond surfaces exposed to the air

The electronic properties of hydrogen- and oxygen-terminated diamond surfaces exposed to the air are investigated by scanning probe microscopy (SPM). The results indicate that for the hydrogen-terminated diamond surface a shallow acceptor above the valence-band-maximum (VBM) appears in the band gap. However, the oxygen-terminated diamond film exhibits a high resistivity with a wide band gap. Based on the density-functional-theory, the densities of states, corresponding to molecular adsorbate in hydrogenated and oxygenated diamond (100) surfaces, are studied. The results show that the shallow acceptor in the band gap for the hydrogen-terminated diamond film can be attributed to the interaction between the surface C--H bonding orbitals and the adsorbate molecules, while for the oxygen-terminated diamond film, the interaction between the surface C--O bonding orbitals and the adsorbate molecules can induce occupied states in the valence-band.