Emitted electrons mean free path

A surface-sensitive version of Mossbauer spectroscopy. Like Mossbauer spectroscopy, this technique is limited to some isotopes of certain metals. After a nucleus is excited by 7-ray absorption, it can undergo inverse /3-decay, creating a core hole. The decay of core holes by Auger processes within an electron mean free path of the surface produces a signal. Detecting emitted electrons as a... 

The information depth of both electron spectroscopies is determined by the inelastic mean-free path of the emitted electrons, which depends on the kinetic energy of the electron in the solid matrix. This dependence is known and has a minimum of about two atomic layers around 25 eV (53). The electron mean-free path is typically larger in oxides than in metals at equal energy, and it is particularly large for zeolites because of their low density. Together with reported ionization cross sections and in the case of AES, Auger decay probabilities, quantitative surface analysis is possible. Compilations of standard spectra are available from which peak energies and sensitivity factors can be obtained (53). 

Such ideal low mean free paths are the basis of FEED, the teclmique that has been used most for detennining surface structures on the atomic scale. This is also the case of photoelectron diffraction (PD) here, the mean free path of the emitted electrons restricts sensitivity to a similar depdi (actually double the depth of FEED, since the incident x-rays in PD are only weakly adenuated on this scale). 

When the electron beam enters the sample, it penetrates a small volume, typically about one cubic micron (10-18m3 ). X-rays are emitted from most of this volume, but Auger signals arise from much smaller volumes, down to about 3 x 10 25m3. The Auger analytical volume depends on the beam diameter and on the escape depth of the Auger electrons. The mean free paths of the electrons depend on their energies and on the sample material, with values up to 25 nm under practical analytical conditions. 

The chemical nature and composition of catalyst surfaces are essential parameters for understanding catalytic reactivity. Electron spectroscopies, mainly Auger Electron Spectroscopy (AES), X-ray Photoelectron Spectroscopy (XPS), Secondary Ion Mass Spectroscopy (SIMS) and Ion Scattering Spectroscopy (ISS) allow such information to be obtained. AES and XPS are most likely to provide meaningful data if the surface region of the solid is homogeneous over a depth several times the inelastic mean free path of the emitted electrons. 

This group of techniques is based upon the analysis of electrons backscattered or emitted from metal surfaces. The shallow escape depths of these particles make their use most suitable for interfacial studies since the information they bear are characteristic only of the near-surface layers on the other hand, the short mean-free paths necessitate a high-vacuum enviromnent. The major limitation has always been the possibility of stractural and compositional changes upon emersion (removal from solution under potential control) and transfer of the electrode into the UHV environment. However, numerous studies have established that the compact layer remains largely unperturbed upon emersion, " unless the emersed layer contains feebly bound non-condensed species. 

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. 

The contrast of a secondary electron image stems from local variations in the number of secondary electrons detected. Secondary electrons are electrons with low energies (< 50 eV). Consequently, their mean free path in the material is low (a few nm). When a sample is tilted at an angle 6 to the plane normal to the beam, the secondary electrons produced at the depth jr cross a smaller distance x cos 6 to the sample surface and are therefore less likely to be absorbed. The amount of secondary electrons emitted is minimal when the electron beam is perpendicular to the surface of the sample, and increases with the sample tilt (Fig. 7.2). The emission of secondary electrons increases very significantly at peaks or points as a greater number of electrons may, in this case, leave the surface all these sharp structures appear bright on a secondary electron image. 

Both electron microprobe analysis and SAM use an electron beam for excitation of the specimen. The difierence between these techniques is in the detection of emitted x-rays in the microprobe technique while SAM measures emitted electrons. For both techniques, the energy of the detected particles is characteristic of the parent atom and thus identifies the atomic species present. The lateral spatial resolution in SAM is superior due to the much shorter mean free path of the emitted energy (electrons). The escape depth of auger electrons is approximately 10 A versus 1000 A in microprobe analysis. This phenomenon makes SAM a highly specific surface analysis technique. 

X-ray photoelectron diffraction is the coherent superposition of a directly photo-emitted electron wave with the elastically scattered waves from near-neighboring atoms. This gives element-specific structural information about the near surface atoms in a single crystal [8-10]. The short inelastic mean free path of the electron waves at the kinetic energies of interest (15 to 1000 eV) leads to surface sensitivity and determination of the atomic geometry of the emitting atom. The known energies of narrow XPS core-level peaks lead to element specificity. The resolution of surface peaks and chemical shifts may even sometimes lead to a chemical state-specific structure determination. 

XPS or AES is extensively used not only to indicate the cleanliness of the sample before transfer, but also to indicate the presence of adsorbates and their oxidation states following electrochemical experiments and transfer back into the UHV environment. In the case of model platinum-based electrocatalysts, the electron spectroscopies have been used to estimate the coverage of the adsorbate metal atoms or the alloy composition. In the case of alloys, or the nucleation and growth of metal adsorbate structures, the techniques give only the mean concentrations averaged over a depth determined by the inelastic mean free path of the emitted electrons. Adsorption and reaction at surfaces often depend on the... 

This equation shows that the tunnel effect is possible. In addition, the emitted carrier has the energy of the order of e E(IX, where X is the mean free path. This energy is a little less than 1 eV, and the positive hole with this energy can react with water to yield H+, and evolve oxygen, after Eq. 6. Two protons react with the two trapped electrons at the micro cracks to evolve hydrogen. 

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