Photoemission synchrotron sources

Fine structure experiments are often carried out with synchrotron sources, since the initial electron state is better defined for photoemission than for electron excitation. When core-hole decay is detected by Auger or secondary electron emission, the technique is surface sensitive. Core-hole decay can also be detected by fluorescence, or by adsorption of the incident photon beam. These methods are not intrinsically surface sensitive, but they are useful when the source atoms are exclusively located at the surface. 

By using circularly polarized X-rays from a synchrotron source the intensity of photoemission becomes dependent on the magnetisation of the sample. This can be used to provide element specific magnetic contrast for imaging magnetic domains whose behaviour can then be studied in real time. 

By using a synchrotron source instead of a He lamp to provide the ultraviolet photons a photon energy can be selected that preferentially favours emission from a particular element by exploiting variations in photoemission cross-section. The partial density of states contribution from each individual element can then be determined. 

Fig. 5. A schematic illustration of an angle-resolved photoemission experiment An incident photon, with wavevector p and polarization E, strikes the sample with polar incidence angles (61p, p) relative to the crystal axes. In practice the light source is generally fixed relative to either the crystal or the detector. However, the ability to vary the photon polarization from synchrotron sources provides a powerful tool for obtaining information on the symmetries of electronic states. By moving the analyzer or the sample (depending on the details of the experimental apparatus), photoelectrons leaving the surface at polar angles (6, ) are collected by the spectrometer the component of their crystal momentum, k, parallel to the sample surface is strictly conserved, allowing accurate determination of the two-dimensional band structure.

As mentioned earlier, the availability of synchrotron source radiation, has added a new dimension to photoemission research. By studying the valence levels with varying energy of the excitation photon, it has become possible to investigate the unoccupied states of the valence region. With the help of a synchrotron source the core and... 

Ultraviolet photoelectron spectroscopy (UPS) is a variety of photoelectron spectroscopy that is aimed at measuring the valence band, as described in sectionBl.25.2.3. Valence band spectroscopy is best perfonned with photon energies in the range of 20-50 eV. A He discharge lamp, which can produce 21.2 or 40.8 eV photons, is commonly used as the excitation source m the laboratory, or UPS can be perfonned with synchrotron radiation. Note that UPS is sometimes just referred to as photoelectron spectroscopy (PES), or simply valence band photoemission. 

From the perspective of this symposium, analysis of the atomic dynamics and electronic structure of surfaces constitutes an even more exotic topic than surface atomic geometry. In both cases attention has been focused on a small number of model systems, e.g., single crystal transition metal and semiconductor surfaces, using rather specialized experimental facilities. General reviews have appeared for both atomic surface dynamics (21) and spectroscopic measurements of the electronic structure of single-crystal surfaces (, 22). An important emerging trend in the latter area is the use of synchrotron radiation for studying surface electronic structure via photoemission spectroscopy ( 23) Moreover, the use of the very intense synchrotron radiation sources also will enable major improvements in the application of core-level photoemission for surface chemical analysis (13). 

Photoemission spectroscopy involves measurement of the energy distribution of electrons emitted from a solid under irradiation with mono-energetic photons. In-house experiments are usually performed with He gas discharge lamps which generate vacuum UV photons at 21.2 eV (He la radiation) or 40.8 eV (He Ila radiation ) or with Mg Ka (hv=1284.6 eV) or A1 Ka (hv=1486.6eV) soft X-ray sources. UV photoemission is restricted to the study of valence and conduction band states, but XPS allows in addition the study of core levels. Alternatively photoemission experiments may be performed at national synchrotron radiation facilities. With suitable choice of monochromators it is possible to cover the complete photon energy range from about 5 eV upward to in excess of 1000 eV. The surface sensitivity of photoemission derives from the relatively short inelastic mean free path of electrons in solids, which reaches a minimum of about 5A for electron energies of the order 50-100 eV. 

Van Buuren et al. [105] performed photoemission and X-ray absorption experiments on Si nanocrystals to determine the TVB and BCB shifts, respectively, as a function of size. The Si nanocrystals were grown in situ at 1700 °C in an Ar gas buffer of 112 mTorr followed by hydrogen exposure to passivate the surface. The resolution of the photoemission and absorption measurements carried out on a synchrotron radiation source were 0.25 eV and 0.05 eV, respectively. They observed a valence band to conduction band shift ratio of 2 1 for all sizes of Si nanocrystals. This is in agreement with various calculations reported for Si nanocrystals [106]. 

Additionally, information on quantum mechanical quantities are usually obtained with a more or less larger experimental or theoretical effort. Being in contrast, the theoretical description of the latter type of angle resolving photoemission experiments as well as the experimental procedure are relatively simple. This method is suitable to be carried out in a laboratory because it does not need a sophisticated experimental setup as synchrotron radiation sources. It may therefore play an important role for a proceeding understanding of the photoemission process. 

X-ray satellites occur from all photoemission fines (not from Auger fines) due to non-monochromatic excitation sources (except when monochromators or synchrotrons are used for excitation). These satellites should be removed as a first data reduction step. The most important satellites occur for Al excitation... 

Today s understanding of the silicon dioxide-silicon interfacial region has come about mostly through photoemission, both on standard UPS and XPS instruments and using synchrotron radiation sources. The transition from bulk silicon to silicon dioxide is basically abrupt with a single... 

As an alternative to the 4f electrons, the 4d and especially 3d core levels, with large cross sections for the excitation with laboratory sources such as AIKa radiation (photon energy 1.486 keV), were very appealing to the experimentalist as possible signals for obtaining, in a fast way, a picture of the underlying electronic structure of the lanthanides. This was especially true for Ce, where - without synchrotron radiation - it was clearly very difficult with X-ray Photoemission Spectroscopy (XPS) to detect the 4f-related intensity (Baer et al. 1978). 

The synchrotron light sources, high resolution spectrometers and modern detection and data acquisition techniques have brought back old absorption spectroscopies into the physics and chemistry of the lanthanides. High quality absorption data from all core levels are available today and are compared with the corresponding X-ray photoemission (XPS) data, which have been collected since the sixties. The final god of both X-ray absorption and X-ray photoemission spectroscopists is to obtain information about the ground state of the solid. A large part of this volume is dedicated to experimental and theoretical aspects of photoemission from the... 

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