Photoelectron spectroscopy electron emission from core

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. 

In atoms and molecules, shakeup satellites, corresponding to internal electronic transitions, are routinely observed using photoelectron and resonant Raman spectroscopy. In particular, shakeup satellites can be observed in the two particle spectrum, i.e., when two holes are left in the final state of an atom after electron emission. Satellite s strength can be strongly enhanced in the presence of a resonant intermediate state. For example, in copper atoms, the incident photon can first excite the core 3p electron to the 4s shell the core hole then decays to the 3d shell through the Auger process (with electron ejected from 3d shell) leaving two 3d holes in the final state [48]. For recent reviews of extensive literature the reader is referred to Refe. [49,50]). 

X-ray photoelectron spectroscopy (XPS), or given its other name Electron Spectroscopy for Chemical Analysis (ESCA), uses X-rays to excite photoelectrons. The emitted electron signal is plotted as a spectrum of binding energies. The photon is absorbed by an atom, molecule or solid leading to ionization and the emission of a core electron. Analysis will reveal the composition from a depth of 2 20 atomic layers and the electronic state of the surface region of the sample. XPS has the ability to identify different chemical states resulting from compound formation, which are revealed by the photoelectron peak positions and shapes. 

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. 

X-ray photoelectron spectroscopy (XPS) operates on the principle of the photoelectric effect, which occurs via a primary excitation process brought about by X-ray-irradiation producing electrons photoelectrons) of discrete energy, containing chemical information regarding the surface analyte. It should be noted that X-rays are only one of many types of excitation sources that can be used to induce emission of electrons for analysis. X-ray photoelectron (XP) spectral peaks (generated by the photoelectrons) are named according to the orbital 1 = 0, 1,2,3... denoted as s, p, d, f...) and spin s = 1/2) quantum numbers of the core levels from which they emanate. The total momentum of the photoelectrons ( / = / x) is included... 

Soft X-Ray Appearance Potential Spectroscopy. In SXAPS the X-ray photons emitted by the sample are detected, normally by letting them strike a photosensitive surface from which photoelectrons are collected, but also— with the advent of X-ray detectors of increased sensitivity— by direct detection. Above the X-ray emission threshold from a particular core level the excitation probability is a function of the densities of unoccupied electronic states. Since two electrons are involved, the incident and the excited, the shape of the spectral structure is proportional to the self-convolution of the unoccupied state densities. 

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