Angle-resolved photoemission energy

Detailed electronic energy-band calculations have revealed the existence of appropriate surface states near the Fermi energy, indicative of an electronically driven surface instability. Angle-resolved photoemission studies, however, showed that the Fermi surface is very curved and the nesting is far from perfect. Recently Wang and Weber have calculated the surface phonon dispersion curve of the unreconstructed clean W(100) surface based on the first principles energy-band calculations of Mattheis and Hamann. ... 

Two rather different techniques that exploit the same underlying phenomenon of coherent interference of elastically scattered low energy electrons are photoelectron diffraction [5] and surface extended X-ray absorption fine structure (SEXAFS) [6,7]. Figure 1.1. shows schematically a comparison of the electron interference paths in LEED and in these two techniques. In both photoelectron diffraction and SEXAFS the source of electrons is not an electron beam from outside the surface, as in LEED, but photoelectrons emitted from a core level of an atom within the adsorbate. In photoelectron diffraction one detects the photoelectrons directly, outside the surface, as a function of direction or photoelectron energy (or both). The detected angle-resolved photoemission signal comprises a coherent sum of the directly emitted component of the outgoing photoelectron wavefield and other components of the same wavefield elastically scattered by atoms (especially in the substrate) close... 

The lower panel of Fig. 4 reproduces angle-resolved photoemission spectra [43] showing the dispersion of the state, i.e. how its BE changes with the angle of emission with respect to the normal. The dotted line in Fig. 3 shows schematically the E(k ) upwards parabolic dispersion of the surface state. The Binding Energy (BE) of the Cu(lll) surface state at the center of the 2D Brillouin Zone (BZ) is —400 meV relative to the Fermi energy. The effective mass for the electrons in this state is obtained from the curvature... 

Angle-Resolved Photoemission. The best experimental technique to resolve the electronic structure of crystals in the momentum-energy space, and, consequently, the Fermi surface, is angle resolved photoemission spectroscopy (ARPES). 

Fig. 3.2. Comparison of theoretical valence-band energies (solid lines cf. Fig. 3.1) with experimental angle-resolved photoemission measurements (after Wang and Klein, 1981 reproduced with the publisher s permission).

Surface analytical techniques such as Auger electron spectroscopy (27), X-ray photoelectron spectroscopy (28), and secondary-ion mass spectrometry (29) have been used to study LB films. Synchrotron radiation is a particularly powerful probe enabling X-ray near-edge structure and extended X-ray absorption fine structure to be measured. Angle-resolved photoemission studies (30) confirmed the existence of a one-dimensional energy band along the (CH2)jc chain in a fatty acid salt film. 

More detailed information on the specific orbitals involved in bonding of the adlayer and the symmetry of the bonding site can be derived from angle resolved photoemission spectroscopy (ARPES). With excitation energies of less than 20 eV, the incident photons excite predominantly valence electrons. The valence electron resonances observed are characteristic of band structure of the substrate and the electron orbitals of the adspecies, whose energy is... 

Given adequately prepared surfaces, angle-resolved photoemission and the various yield spectroscopies have been used to investigate filled and empty surface states, respectively. Results of angle-resolved photoemission measurements have been published by Knapp and Lapeyre [181], Williams et al. [182], Knapp et al. [183] and Huijser et al. [184], A typical set of angle-resolved photoelectron energy distributions (AREDCs) due to Huijser et al. [184] is shown in Fig. 16, in which four structures labelled B , SM S2 and B2 are observed. They are ascribed to emission from filled intrinsic states since they disappear on exposure to 10s L of H2. As we shall see below, B , S and S2 are primarily As-derived, while B2 is mainly a Gas-like state bonded to Asp-states. 

Especially angle resolved photoemission is fundamental for the determination of electronic properties. In conjunction with a mnability of the photon energy a direct assignment of the orbital nature of the states which hybridize in the valence band can be obtained [19, 20]. 

Fig. 12a, b. Angle resolved photoemission spectra of CO adsorbed on Ni(100) at a photon energy of 28 eV. (a) Nearly p-polarized light (b) s-polarized light [76Smi]. 

Figure 9.3 Comparison ofthe calculated energy bands of copper (curves) with experimental data (points) measured by angle-resolved photoemission (after [1]).

---