Elastic-peak electron spectroscopy

DEPES Directional elastic peak electron spectroscopy... 

We turn now to determinations of IMFPs from elastic-peak electron spectroscopy (EPFS) experiments. This approach, which combines measurements of the elastically backscattered intensity from a surface and MC simulations for the same experimental conditions, provides a convenient means for obtaining IMFPs for many materials that can be compared with values calculated from optical data [49, 63-66]. The samples are typically sputtered both to clean the surfaces and to ensure that crystalline order is substantially removed (in order to avoid diffraction effects) it is important to choose an ion species and a large-enough ion energy so that the depth of the amorphized region is at least as large as the EPES sampling depth [67]. 

Figure Bl.25.12. Excitation mechanisms in electron energy loss spectroscopy for a simple adsorbate system Dipole scattering excites only the vibration perpendicular to the surface (v ) in which a dipole moment nonnal to the surface changes the electron wave is reflected by the surface into the specular direction. Impact scattering excites also the bending mode v- in which the atom moves parallel to the surface electrons are scattered over a wide range of angles. The EELS spectra show the higlily intense elastic peak and the relatively weak loss peaks. Off-specular loss peaks are in general one to two orders of magnitude weaker than specular loss peaks.

Figure 7.26. Electron energy-loss spectroscopy (EELS) spectra. Shown (top) is a representative EELS spectrum of a nickel oxide sample. A typical EELS spectrum shows a zero-loss peak that represents the unscattered or elastically scattered electrons, the near-edge fine structure (ELNES), and extended energy-loss fine structure (EXELFS). Also shown (bottom) are the fingerprint regions of an EELS spectrum, just beyond the core-electron edges, which provide information regarding the detailed bonding and chemical environment of the desired element.

Quantitative AES analysis was carried out using a standardization technique developed in our laboratory (55). Namely, a dry, thin layer of sulfate deposit was obtained on the Me(lll) template from a 0.3 M Na2S04 solution and subsequent water evaporation. This Na2S04 covered Me(lll) was used as a standard for work with monolayer (bi)sulfate adlattices. The procedure involved a comparison of the peak-to-peak (p/p) intensities of sulfur and oxygen at 131 eV and 516 eV, respectively, relative to the Me p/p (Au 69 eV, Rh 302 eV, Pt 64 eV) intensity of the clean Me(l 11) sample. As mentioned above, the chemical state of the adsorbate was interrogated by the Core Level Energy Loss Spectroscopy. The loss energies reported here were measured relative to the electron elastic peak of 500 eV electrons. 

Elastic tunneling spectroscopy is discussed in the context of processes involving molecular ionization and electron affinity states, a technique we call orbital mediated tunneling spectroscopy, or OMTS. OMTS can be applied readily to M-I-A-M and M-I-A-I -M systems, but application to M-A-M junctions is problematic. Spectra can be obtained from single molecules. Ionization state results correlate well with UPS spectra obtained from the same systems in the same environment. Both ionization and affinity levels measured by OMTS can usually be correlated with one electron oxidation and reduction potentials for the molecular species in solution. OMTS can be identified by peaks in dl/dV vs bias voltage plots that do not occur at the same position in either bias polarity. Because of the intrinsic... 

Inelastic electron tunnelling spectroscopy (lETS) has been used to study some silanes on aluminium oxide. The technique records vibrational spectra of an absorbed monolayer. Silanes can be applied to the oxidised metal from solution or vapour, and devices are completed by evaporation of a top electrode which is usually of lead, because of its superconductivity. The device is cooled to the temperature of liquid helium (4.2 K) to minimise thermal broadening. Most electrons (>99%) pass through the device elastically, but a small number excite vibrational modes. It is these that are detected and displayed as a spectrum. Both IR and Raman modes can be observed the selection rule for lET spectroscopy is one of orientation, in that bonds which are aligned perpendicular to the surface give the most intense peaks. 

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