Electron escape depth

Local surface structure and coordination numbers of neighbouring atoms can be extracted from the analysis of extended X-ray absorption fine structures (EXAFS). The essential feature of the method22 is the excitation of a core-hole by monoenergetic photons modulation of the absorption cross-section with energy above the excitation threshold provides information on the distances between neighbouring atoms. A more surface-sensitive version (SEXAFS) monitors the photoemitted or Auger electrons, where the electron escape depth is small ( 1 nm) and discriminates in favour of surface atoms over those within the bulk solid. Model compounds, where bond distances and atomic environments are known, are required as standards. 

Fig. 5. Electron escape depth as a function of electron energy. The energies of several photon sources are shown.

For calculating atomic concentration ratios of the elements, photoionization cross-section by Scofield (4) and apparatus function by VuUi (5) were adopted. Electron escape depth (a) is determined by an experimental equation A =e0 7 (where E is kinetic energy of the electron) proposed by Hirokawa, et. al. (6). 

Fig. 1. Electron escape depths as a function of electron energy, o. in the noble metals. Ref. (3), (4), and (5)...

The propagation of electromagnetic waves in bulk solid materials and in molecules is well understood. This understanding is appropriate for bulk EXAFS, for instance, but not necessarily for surface studies. The situation of a solid surface has only been studied recently in terms of atomic-scale phenomena, as is required by techniques such as photoemission. Although photons typically penetrate to a depth of about 1 fim into the metal surfaces of interest, the only important photon-induced excitations occur within the electron escape depth of about 5 to 10 A. Therefore, the shape of the electromagnetic fields should be known in this near-surface region of a few atomic layers. 

Figure 1- Electron escape depths vs. energy. (Adapted from Ref. 16 )...

As such, it competes directly with X-ray fluorescence (XRF) but it is not limited by the dipole operator selection rules. All energetically allowed transitions are observed in an Auger electron spectrum. In addition, the electron escape depth is also a few tens of angstroms, unlike XRF where typical escape depths are on the order of tens of thousands of angstroms. 

Fig. I. Photoelectron mean free path for inelastic scattering in Si and Au in the energy range of interest for XANES spectra. The data were obtained from the measurement of electron escape depth in photoemission experiments...

Interfaces can also be studied through the use of the electron escape depth in photoemission experiments. The chemistry and formation of interfaces can be observed by growing or depositing material to form the desired interface This requires that the deposition or growth be performed either in-situ or in a vacuum interlocked system to keep extraneous contamination to a minimvim. If the interface is within the electron escape depth variable energy x-ray photoemission experiments can be performed to yield interface chemistry. Later on, these methods applied to the silicon dioxide-silicon interface and metal-III-V interfaces will be discussed in some detail. Because the above methods depend on the electron escape depth phenomenon, they are non-destructiva However, studies can only be made on model systems. 

The electron involved in the donor-acceptor interaction need not originate from the actual surface layer but may well come from the interior of the bilipid membrane, for example, from intramembrane proteins. The electron escape depth in organic materials is comparable to intermolecular distances. 

S. Hino and H. Inokuchi, Electron Escape Depths of Organic Solids. II. The Energy Dependence of Naphthacene and Perylene Films, J. Chem. Phys. 70, 1142-1146 (1979). 

Fig. 5.8. Universal curve of electron escape depth (average distance between inelastic collisions (A)) vs. kinetic energy (eV)...

Figure 3.20 Electron escape depth A and mean free path A of electrons. Quantitative analysis...

---