Photoelectron spectroscopy electron emission from valence

In addition to using X-rays to irradiate a surface, ultraviolet light may be used as the source for photoelectron spectroscopy (PES). This technique, known as ultraviolet photoelectron spectroscopy (UPS, Figure 7.38), is usually carried out using two He lines (Hel at 21.2 eV and Hell at 40.8 eV), or a synchrotron source. This technique is often referred to as soft PES, since the low photon energy is not sufficient to excite the inner-shell electrons, but rather results in photoelectron emission from valence band electrons - useful to characterize surface species based on their bonding motifs. It should be noted that both UPS and XPS are often performed in tandem with an Ar" " source, allowing for chemical analysis of the sample at depths of < 1 J,m below the surface. 

The electron affinity can also be deduced from the measurement of the spectrum of the photoelectron emission with monochromatic UV light. This technique is ultra-violet (UV) photoelectron emission spectroscopy (or UV photoemission spectroscopy or UPS). The UPS technique involves directing monochromatic UV light to the sample to excite electrons from the valence band into the conduction band of the semiconductor. Since the process occurs near the surface, electrons excited above the vacuum level can be emitted into vacuum. The energy analysis of the photoemitted electrons is the photoemission spectrum. The process is often described in terms of a three step model [8], The first step is the photoexcitation of the valence band electrons into the conduction band, the second step is the transmission to the surface and the third step is the electron emission at the surface. The technique of UPS is probably most often employed to examine the electronic states near the valence band minimum. 

X-ray and electron spectroscopies are very useful techniques to study the electronic state and chemical bonding of various kinds of functional materials, such as ceramics and alloys. Since the leading achievement by Siegbahn et al. x-ray photoelectron spectroscopy (XPS) is known to be very efficient for chemical state analysis of matters. They provide information not only on chemical components but also that on the valence electronic state and the chemical bonding of atoms constructing the materials. The direct information on the density of state (DOS) for solid state material can be obtained from XPS of the valence state region. Figure 1 schematically illustrates the relationship between the electronic state of matter and photoelectron spectrum as well as x-ray emission and absorption spectroscopies, and also the characteristics of these spectroscopies. 

Photons as probes may also cause the emission of electrons from the sample, and the kinetic energy distribution of these photoelectrons can be recorded and analysed. This technique is called photoelectron spectroscopy (PES) in general, but if the photons are in the ultraviolet range, UPS, or in the X-ray range, XPS. Photoelectron spectroscopy permits direct examination of electronic orbitals of atoms by providing information about the electrons in both the valence bands and the core levels of the constituent elements of solids. Under suitable conditions the electronic states of bulk and surface atoms can be distinguished. 

A particularly useful variety of UPS is angle-resolved photoelectron spectroscopy (ARPES), also called angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) [, 62]. In this technique, measurements are made of the valence band photoelectrons emitted into a small angle as the electron emission angle or photon energy is varied. This allows for the simultaneous determination of the kinetic energy and momentum of the photoelectrons with respect to the two-dimensional surface Brillouin zone. From this information, the electronic band structure of a single-crystal material can be experimentally determined. 

Ultraviolet photoelectron spectroscopy is capable of providing chemical state and electronic structure information from materials. However, due to the complex nature of the density-of-states (DOS) in the valence band, it is more difficult to extract this information, as compared to XPS, usually requiring band-structure calculations and other spectroscopies. By observation of the onset of photoelectron emission, work function measurements may be made using UPS. Like XPS, UPS is non-destructive. However, UPS cannot typically provide quantitative information. 

A.10.2.1 XPS and UPS X-ray Photoelectron Spectroscopy (XPS) is a technique capable of providing the elemental composition of the outer l-5 nm from any solid, although insulators are difficult, to detection limits down to 0.1% (detection limits are element dependent) with some spedation information also available. All elements from Li to U are detectable. The spatial resolution can be down to 2 pm. This technique does so by directing au x-ray beam (Al-ka is most often used) at the solid of interest. This induces core electron emission (valence electrons are also produced). Elemental identification is made possible as the core electron energies are element/level specific. No prior sample preparation is needed, but analysis must be carried out under UHV conditions. 

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