Surface concentrations escape depth data

Both the calculated photoelectron ionization and escape depth data of Scofield (11) and Penn (12) are invaluable in estimating surface concentrations from Eq. (8). More recently, experimental cross section data have been reported by Thomas and his group (13) the reported data are relative to the F(ls) peak taken as unity. There are clearly examples where Scofield s calculated cross section values are at variance with the experimentally determined ones the variation is particularly noticeable when we consider outer levels, e.g., for K(2p) there are serious discrepancies, whereas the K(2s) data are acceptable. 

As with XRF, electron microscope-based microanalysis is relatively-insensitive to light elements (below Na in the periodic table), although this can be improved upon with developments in thin-window or windowless detectors which allow analysis down to C. It is better than XRF because of the high vacuum used ( 10-8 torr), but a fundamental limitation is the low fluorescent yield of the light elements. As with XRF analysis it is surface sensitive, since the maximum depth of information obtained is limited not by the penetration of the electron beam but by the escape depth of the fluorescent X-rays, which is only a few microns for light elements. In quantitative analysis concentrations may not add up to 100% because, if the surface is not smooth, some X-rays from the sample may be deflected away from the detector. It may be possible in such cases to normalize the concentration data to 100% if the analyst is certain that all significant elements have been measured, but it is probably better to repeat the analysis on a reprepared sample. 

The significance of the development of photoelectron spectroscopy over the last decade for a better understanding of solid surfaces, adsorption, surface reactivity, and heterogeneous catalysis has been discussed. The review is illustrative rather than exhaustive, but nevertheless it is clear that during this period XPS and UPS have matured into well-accepted experimental methods capable of providing chemical information at the molecular level down to 10% or less of a monolayer. The information in its most rudimentary state provides a qualitative model of the surface at a more sophisticated level quantitative estimates are possible of the concentration of surface species by making use of escape depth and photoionization cross-section data obtained either empirically or by calculation. 

Elemental Sensitivities. The number of counts collected in a data channel, of energy width AE, is directly proportional to the concentration of target atoms located within that corresponding depth interval. Upon calibrating a facility for a given isotope, concentrations can be measured for that Isotope in subsequent samples, Independent of the matrix, the concentration level, or location within the depth that induced particles can escape the sample surface. 

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