Photoemission spectroscopy experimental techniques

The unoccupied electronic states of a solid can be experimentally explored by different techniques. The most commonly used are inverse photoemission, where low-energy electrons impinge on the surface of the solid, and the photon-based techniques ellipsometry, NEXAFS and constant-initial-state spectroscopy. Results derived from inverse photoemission spectroscopy might be questionable unless low-energy electrons (c. 10-20 eV) and low beam currents are used as in LEED... 

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). 

In most cases the experimental techniques used to study surface phenomena do not seem to yield consistent values for the surface segregation energies. One important exception is the special case of an atom of atomic number Z+1 in a host of atoms of atomic number Z, where the surface segregation energy may in fact be extracted with a high degree of accuracy from X-ray photoemission spectroscopy (XPS) measurements of surface core-level shifts (SCLS) [39]. In contrast they may be calculated quite accurately by modern first-principles methods [18,25,40]. 

Time dependent fluorescence depolarization is influenced by the exciton annihilation which occurs in confined molecular domains . Photoemission results from singlet exciton fusion as shown by the excitation intensity dependence which occurs in anthracene crystals. Reabsorption of excitonic luminescence is an effect which has been shown to occur in pyrene crystals. The dynamics of exciton trapping in p-methylnaphthalene doped naphthalene crystals involves phonon assisted detrapping of electronic energy. Ps time resolved spectroscopy was the experimental technique used in this work. 

The distinct advantage of scanning tunneling spectroscopy (STS), compared to conventional experimental techniques like photoelectron spectroscopy and inverse photoemission, is the high lateral resolution. Additionally, electronic states can be investigated in a single measurement on both sides of the Fermi level which cannot be carried out in photoemission techniques photoemission allows to determine occupied states only, inverse photoemission empty states only. 

The experimental papers cover the various spectroscopic techniques and a few deal with special materials. The introductory chapter (62) by Baer and Schneider presents an overview of this field and helps tie the various aspects together that are reviewed in detail in the remaining chapters of the volume. Photoemission studies (UPS - ultraviolet photoemission spectroscopy, and XPS - X-ray photoemission spectroscopy) on various materials are discussed by Campagna and Hillebrecht (chapter 63)- intermetallic compounds, by Lynch and Weaver (chapter 66)— cerium and its compounds, and by Hiifner (chapter 67) - chalcogenides. Other experimental techniques covered include BIS (bremsstrahlung isochromat spectroscopy) by Hillebrecht and Campagna (chapter 70), X-ray absorption and X-ray emission by Rohler (chapter 71) and inelastic electron scattering by Netzer and Matthew (chapter 72). 

Although we confine ourselves to a small area, this topic is still an essential part of physical chemistry and chemical physics, and, more importantly, used in all major areas of chemistry and physics as a tool of determining structures and detecting chemical species. Experimentally, optical excitations can be measured by photoabsorption or photoemission spectroscopy. Therefore, this is the corresponding technique to theoretically calculated spectra that we will deal with in the following. 

With the availability of intense tunable radiation in the range firom ultraviolet to hard X-rays from synchrotrons, powerful new experimental techniques have been developed to probe the structural and electronic properties of solids and surfaces. In particular, angle-resolved photoemission gives information about the electronic properties in the valence bands of solids while core level spectroscopy provides an element-specific spectroscopic tool. 

Some of the previously described techniques, such as TEM and photoemission spectroscopy, require to be carried out in vacuum this environment may introduce artefacts due to the complete desolvation of the electrode coating. Luckily, some modem experimental setups, such as environmental SEM, allow the characterisation of samples at relatively high pressure values. 

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