Escape depth, average

The sample specimen is rotated normal to the direction of the slit entrance to the CHA in order to adjust the surface-sensitivity of the analyte being sampled. Figure 8 shows a schematic depicting the relation between the IMFP, X, and the average escape depth of the photoelectrons ... 

A drawback of the AES method (12) is that it samples a weighted average over several outer layers of the sample, depending on the escape depth of the measured electrons. A comparison of results of this technique and chemisorp-tive titration (13,14), in which chemisorption takes place selectively on one of the constituent atoms, has been made for the Pt-Sn system. This has provided information on the concentration profiles near the surface. 

Surface segregation depends strongly on the mobility of the atoms in the solid and annealing of the Ir-Cu sample has indeed dramatic influence on composition and surface structure. As by a good choice, the composition has been determined with a combination of ISS and XPS measurements. The different surface sensitivities of the two methods (topmost layer for ISS versus an average over some surface layers for XPS determined by the mean escape depth of the photo-electrons) can be used to find the actual composition of the surface layers (Fig. 11). 

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

ARXPS was used as an in-depth analysis, and was carried out by varying the take-off angle (a) and taking into account the relation between the escape depth d) and the photoelectron mean free path (X) d < 3 A, sin a. For simplicity, the average value chosen for the mean free path of the C Is photoelectrons excited with Mg Ka (KE 969 eV) in polymers was A, = 3.2 mn [29-32], which enables us to obtain chemieal information from maximum depths between 2.5 and 9.3 run by varying the analysis angle between 15 and 75°. For photoelectrons of different kinetic energy (O Is, S 2p, N Is, P 2p,...), the mean free path varies less than 0.5 nm, which is considered within experimental errors for polymers. 

Although it is possible to use XPS to distinguish between homogeneous and inhomogeneous mixed valence (Wertheim et al. 1978, 1980), when quoting average valences one has always to take into account that the electronic properties of the surface of both stable and mixed valence intermetallic compounds may differ from the bulk especially whenever the 4f level lies close to the Fermi level. The role of the escape depth, i.e, dependence of the mean-free-path of the photoelectrons from their kinetic energy, must be taken into account. 

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