Escape depth, for electrons

Use electron-yield detection. In the hard X-ray range, the escape depth for electrons is much less than the absorption depth for X-rays, so there is little thickness effect because the electrons are effectively probing a part of the sample that is thin compared with the absorption depth. This method can be useful for standards and other highly-concentrated samples. However, if the sample is illuminated at grazing incidence, there can be an over-absorption effect as well because then the X-rays do not penetrate much farther than where the electrons... 

The intensity of a given photoelectron line is proportional to the X-ray flux, cross-section for exciting the particular level, density of the particular atom in the lattice x, the escape depth for electrons of the resulting kinetic energy, asymmetry factor in the angular distribution of the photoionization event, transmission of the analyzer, which includes the acceptance angle and area, and the efficiency of the electron detector. In addition, there are several secondary factors, some of which depend on the matrix. The intensity of line is given by... 

UHV techniques are usually classified in terms of the electron/photon method, as is shown in Table 2.3 which lists the common electron bombardment and emission techniques that have been employed in electrochemical studies. A detailed description of UHV surface analysis techniques is beyond the scope of this book there are many excellent reference texts that can be consulted for this purpose (see further reading list). It is sufficient to note that methods involving electron bombardment or emission are inherently surface-sensitive as a result of the low pathlength, or escape depth, of electrons in condensed media. In addition, Table 2.3 briefly describes the type of information each method provides. 

The escape probability for electrons from the target is related exponentially to their depth. 

Pig. 3. Schematic of generalized form of escape depth versus kinetic energy for electrons. [Also illustrated are the different effective sampling depths for electrons corresponding to differing escape depths ( s).l... 

It is clear that the difficulties associated with angular dependence, instrument factors, and atom densities of A and B involved in absolute intensity measurements can be obviated by studying ratios of intensities of the same levels as a function of angle for overlayers of different thickness d. In this way we may obtain directly escape depths for photoemitted electrons in material A and in general since the kinetic energy will be different for photoemission from the core levels of A and B this will yield two values XA and XA. If the experiment is now repeated with different core levels of sample A and with a different X-ray source we may start to build up a picture of escape depths as a function of kinetic energy. 

The primary objective of many of the incident radiations listed in Table 1 is ionisation and, in some cases, arrangements can be made for energy deposition to be concentrated in regions close to the surface. Small escape depths for emitted electrons mean, in any event, that these carry information mainly about species in the topmost few layers of the solids. The high sensitivities attainable in energy analysis of such electrons means that information can be obtained without the need to... 

A companion field, Auger electron spectroscopy (AES), was developed simultaneously. AES does not provide chemical species information, only elemental analysis, as we will see. Since the electrons ejected in these two techniques are of low energy and the probability of electron interaction with matter is very high, the electrons cannot escape from any significant depth in the sample. Typical escape depths for XPS and AES electrons range from 0.5 to 5 nm for materials. The phenomenon is therefore confined to a few atomic layers, combined or otherwise, which are at the surface of the sample and provides a method of surface analysis. 

Figure 4 Primary excitation volume and escape depth for Auger electrons. (Reproduced with permission from ULVAC-PHI.)...

Measurements of this effect for a SmS, EuS and YbS are shown in fig. 17 (Martensson et al. 1982). They have been performed with a photon energy of 70 eV which brings out specifically the electronic properties of states in the surface layer because of the small escape depth of electrons with kinetic energies in that range. The separation of the bulk and the surface contribution to the spectra can be... 

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