Electron photoemission measurements

The discussion of section 3.2 shows that photoemission spectra for a- and y-Ce are not simple. In principle, the corresponding photoemission spectra for Ce vapor would be simpler than those of the metal because there are fewer valence electrons to respond to the photohole. Valence-electron photoemission measurements of Ce vapor have not as yet been carried out. Studies of small clusters of Ce metal have, however, been reported by Fujimori et al. (1985). 

In this section the electronic structure of metal/polymcr/metal devices is considered. This is the essential starting point to describe the operating characteristics of LEDs. The first section describes internal photoemission measurements of metal/ polymer Schottky energy barriers in device structures. The second section presents measurements of built-in potentials which occur in device structures employing metals with different Schottky energy barriers. The Schottky energy barriers and the diode built-in potential largely determine the electrical characteristics of polymer LEDs. 

Electron work functions of metals in solution can be determined by measurements of the current of electron photoemission into the solution. In an electrochemical system involving a given electrode, the photoemission current ( depends not only on the light s frequency v (or quantum energy hv) but also on the potential E. According to the quantum-mechanical theory of photoemission, this dependence is given by... 

Extended X-ray absorption fine structure (EXAFS) and photoemission measurements carried out on the clusters ranging from isolated atoms to aggregates large enough to acquire bulk metal properties, is also could be a measure for the electronic properties [40]. Copper shows a... 

Since the gold chains are located at the surface of the sample, the wave vectors of the electronic states linked to the chains lie in the surface plane. This wave vector, 11 = sin is strictly conserved in the photoemission measurements. 

Mazin, 1.1. and S. L. Molodtsov. 2005. Electrons and phonons in YbC6 Density functional calculations and angle-resolved photoemission measurements. Phys. Rev. B 72 172504-1-4. 

Figure 2 also shows a d-band, arising from the four nickel atoms with d electrons explicitly included, extending downward from about -0.5 a.u. for the clean surface, adsorbed CH and coadsorbed CH and H cases. In a Ni atom, for this basis, the average d orbital energy is -0.44 a.u., a value close to the Hartree-Fock result. Photoemission measurements position the d ionization peaks of nickel near the Fermi level, a result also obtained by most density functional treatments of nickel clusters. Application of Koopmans theorem would therefore suggest that the present d-ionization... 

To a first approximation angle integrated photoemission measures the density of occupied electronic states, but with the caveat that the contribution of a given state to the spectrum must be weighted by appropriate ionisation cross sections. Comprehensive tabulations of ionisation cross sections calculated within an independent electron framework are available [8]. At X-ray energies cross sections for ionisation of second and third row transition metal d states are often very much greater than for ionisation of O 2p states, so that valence band X-ray photoemission spectra represent not so much the total density of states as the metal d partial density of states [9],... 

Brodsky and Frumkin estimated the possible thermoemission rates (from electrodes into aqueous solutions), using the Richardson-Sommerfeld equation and the electronic work function for the metal-water system, determined from photoemission measurements. They have shown that in the range of potentials typical of cathodic reactions in aqueous solutions the emission rates should be very small and need not ensure cathodic evolution of hydrogen via the thermoemission stage, i.e., via the intermediate formation of hydrated electrons. 

An interesting study in which the problems associated with differences in magnetic properties between surface and bulk of the films were addressed was made by Bona et al. (1986). These authors used Kerr-rotation measurements together with threshold photoemission measurements with polarized electrons to show that the... 

Surface sensitivity is an intrinsic property of photoemission measurements. The incident light penetrates far into the solid, but the escape depth of excited electrons is very short (Fig. 5), although there are local variations related to direction-dependent band structure effects. Surface sensitivity can be further enhanced by appropriate choice of experimental parameters such as photon energy, angles of incidence and emission, etc., which take advantage of selection rules favouring surface processes. 

Ceo fullerides exhibit several interesting phenomena related with the presence of strong correlations such as high temperature superconductivity or antiferromagnetism. The existence of non-conventional behaviors can be anticipated from the fact that both the electron-phonon and electron-electron interactions are, respectively, comparable and much larger than the narrow bandwidth predicted by standard electronic structure calculations (a few hundreds of meV). These systems are therefore close to a metal-insula-tor transition of the Mott-Hubbard type and the validity of the adiabatic approximation, assumed in most electronic structure calculations, can be questioned. From the experimental point of view the band derived from the molecular LUMO is usually seen as a quite broad feature, much wider that the theoretical estimates, in photoemission experiments. However, the observation of the band dispersion has proven elusive in ARPES studies until very recently. In a recent joint experimental and theoretical paper, Yang et al. [158] have reported the first photoemission measurement of the band dispersion for a K-doped Qo monolayer deposited on a Ag(lll) substrate. The results have been compared with ab initio calculations performed with SIESTA. In those calculations the Ag substrate was modeled by a slab con-... 

Apart from transistors, several other solid state devices have been discussed [78], like junctions, photon and electron beam switches and various kinds of sensors. One property of diamond which has stimulated considerable interest in the recent years is the negative electron affinity (NEA) of suitably prepared surfaces [78,80]. The electron affinity, of a material is defined as the difference between the energy of a free electron in vacuum and the bottom of the conduction band Fyac - E. In Fig. 8 the electronic bands of p-doped clean and H-terminated (111) diamond surfaces near the surface are depicted, based on the results of UV-photoemission measurements. For the H-terminated surface, the electron affinity becomes negative once an electron is injected into the conduction band from a suitable contact or by UV excitation, it will easily leave the crystal and be emitted into vacuum. This effect, which is also observed on monohydride terminated (100) surfaces, is not unique to diamond but was also observed in a few other semieonductors with high band gaps [80]. Apart from a scientific interest, the NEA of diamond makes it an attractive eandidate for the replacement of thermionic emitters as electron beam sourees and as a miniature electron emitter for field emission displays. 

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