Surface Concentrations from Photoelectron Intensity Data

Calculation of Surface Concentrations from Photoelectron Intensity Data 

In order to estimate the photoelectron yield from a chemisorbed layer we follow the approach of Henke (S), applied by Madey et al. (9), and modified by Carley and Roberts (10). A monoenergetic beam of x rays strike a surface at an angle 0 to the surface normal the photoelectrons, assumed to be generated in a layer of depth X and thickness dx, escape from the sample to a detector positioned at an angle with respect to the surface normal. The differential probability dP of absorption of x rays at depth X within the thickness dx is 

For a monolayer containing a atoms cm the probability of x-ray absorption by the monolayer is given by 

If we divide Eq. (6) by Eq. (4), then the photoelectron yield from the mono-layer (F ) relative to the total yield from the substrate (Fj) is given by 

Equation (8) therefore allows us to use directly tabulated subshell photo-ionization cross sections (p) instead of mass absorption coefficients (a). F is the integrated photoelectron signal from an appropriate subshell of the monolayer adatom Yg the integrated signal from the relevant subshell of the substrate which is not simply the area of the core-level peak p and p. 

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III. Calculation of Surface Concentrations from Photoelectron Intensity Data... 

There arc two principal ways in which surface concentration data may be obtained by the evaluation of XPS intensities. One relies on a first-principle description of photoelectron emission from a solid surface, the other on empirically determined elemental. sensitivity factors (cf. Chapter 4 and 5). Either approach can be used in its simplest form to estimate elemental surface concentration ratios from XPS intensity ratios for a catalyst surface, provided the sample is homogeneous and isotropic, i.e.. all elements are uniformly distributed in the surface layer sampled by XPS. However, a heterogeneous catalyst is in fact just that—heterogeneous it can be multipha.se and of complex structure. Operative words here are porosity, inner and outer surface, texture, segregation, etc. Nevertheless the simple procedures have been used extensively in catalyst characterization studies for straightforward interpretation of XPS in-... 

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