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Structure and Bonding, Volume 45: Breakdown of the one-electron pictures in photoelectron spectra
Most instrument parameters are constant while others depend on the choice of experimental settings. The full width at half maximum FWHM values are useful indicators of chemical state changes and physical influences. However, the FWHM also depends on the detector, and can also increase due to the sample getting charged. Chemical shift values depend on the degree of electron bond polarization between nearest neighbor atoms.
A specific chemical shift is the difference in BE values of one specific chemical state versus the BE of one form of the pure element, or of a particular agreed-upon chemical state of that element. Component peaks derived from peak-fitting a raw chemical state spectrum can be assigned to the presence of different chemical states within the sampling volume of the sample.
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A hemispherical electron energy analyzer is generally used for applications where a higher resolution is needed. An ideal hemispherical analyzer consists of two concentric hemispherical electrodes inner and outer hemispheres held at proper voltages. It is possible to demonstrate that in such a system, i the electrons are linearly dispersed along the direction connecting the entrance and the exit slit, depending on their kinetic energy, while ii electrons with the same energy are first-order focused. After some algebra, the following expression can be derived for the potential:.
From this equation, we can calculate the potential difference between the two hemispheres, which is given by:.
The instrumental energy resolution of the device depends both on the geometrical parameters of the analyzer and on the angular divergence of the incoming photoelectrons:. In order to reduce the acquisition time per spectrum, the so-called snapshot or fixed mode has been introduced. This mode exploits the relation between the kinetic energy of a photoelectron and its position inside the detector. If the detector energy range is wide enough, and if the photoemission signal collected from all the channels is sufficiently strong, the photoemission spectrum can be obtained in one single shot from the image of the detector.
Since the relevant information, in photoemission spectroscopy, is contained in the kinetic energy distribution of the photoelectrons, a specific device is needed to energy-filter the electrons emitted or scattered by the sample. Electrostatic monochromators are the most common choice. The so-called cylindrical mirror analyzer CMA is mostly used for checking the elemental composition of the surface.
It consists of two co-axial cylinders placed in front of the sample, the inner one being held at a positive potential, while the outer cylinder is held at a negative potential. Only the electrons with the right energy can pass through this set-up and are detected at the end. The count rates are high but the resolution both in energy and angle is poor. A breakthrough has been actually brought about in the last decades by the development of large scale synchrotron radiation facilities.
Here, bunches of relativistic electrons kept on a circular orbit inside a storage ring are accelerated through bending magnets or insertion devices like wigglers and undulators to produce a high brilliance and high flux photon beam. The main advantages of using synchrotron light are.
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The highest spectral brightness and narrowest beam energy dispersion is attained by undulators, which consist of periodic array of dipole magnets in which the electrons are forced to wiggle and thus to emit coherent light. Besides the high intensity, energy tunability is one of the most important advantages of synchrotron light compared to the light produced by conventional X-ray sources. In fact, a wide energy range from the IR to the Hard X-ray region, depending on the energy of the electron bunch is accessible by changing the undulator gaps between the arrays.
Continuous energy spectra available from a synchrotron radiation source allows selection of photon energies yielding optimum photoionization cross-sections appropriate for probing a particular core level. The high photon flux, in addition, makes it possible to perform XPS experiments also from low density atomic species, such as molecular and atomic adsorbates. Electrons can be detected using an electron multiplier , usually a channeltron. This device essentially consists of a glass tub with a resistive coating on the inside.
A high voltage is applied between the front and the end. An electron which enters the channeltron is accelerated to the wall, where it removes more electrons, in such a way that an electron avalanche is created, until a measurable current pulse is obtained. From the theoretical point of view, the photoemission process from a solid can be described with a semiclassical approach, where the electromagnetic field is still treated classically, while a quantum-mechanical description is used for matter.
The one—particle Hamiltonian for an electron subjected to an electromagnetic field is given by:. Fermi's Golden Rule uses the approximation that the perturbation acts on the system for an infinite time. This approximation is valid when the time that the perturbation acts on the system is much larger than the time needed for the transition. The photoemission event leaves the atom in a highly excited core ionized state, from which it can decay radiatively fluorescence or non-radiatively typically by Auger decay.
Besides Lorentzian broadening, photoemission spectra are also affected by a Gaussian broadening, whose contribution can be expressed by.
Three main factors enter the Gaussian broadening of the spectra: the experimental energy resolution, vibrational and inhomogeneous broadening. The first effect is caused by the non perfect monochromaticity of the photon beam -which results in a finite bandwidth- and by the limited resolving power of the analyzer.
The vibrational component is produced by the excitation of low energy vibrational modes both in the initial and in the final state. Finally, inhomogeneous broadening can originate from the presence of unresolved core level components in the spectrum. In a solid, also inelastic scattering events contribute to the photoemission process, generating electron-hole pairs which show up as an inelastic tail on the high BE side of the main photoemission peak. In some cases, we observe also energy loss features due to plasmon excitations. This can either a final state effect caused by core hole decay, which generates quantized electron wave excitations in the solid intrinsic plasmons , or it can be due to excitations induced by photoelectrons travelling from the emitter to the surface extrinsic plasmons.
Temperature-dependent atomic lattice vibrations, or phonons , can broaden the core level components and attenuate the interference patterns in an XPD X-Ray Photoelectron Diffraction experiment. From Wikipedia, the free encyclopedia. This section is in list format, but may read better as prose.
You can help by converting this section , if appropriate. Editing help is available. November This section needs expansion. You can help by adding to it. Al Nuclear Physics. Bibcode : NucPh The Journal of Chemical Physics. Bibcode : JChPh.. Analytical Chemistry, XPS International, Inc. Retrieved 8 December Erskine Journal of Electron Spectroscopy and Related Phenomena.
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