Experimental Aspects of XPS

An XPS spectrometer contains an X-ray source - usually Mg Ka (1253.6 eV) or A1 Ka (1486.3 eV) - and an analyzer which, in most commercial spectrometers, is hemispherical in design. In the entrance tube, the electrons are retarded or accelerated to a value called the pass energy , at which they travel through the hemispherical filter. The lower the pass energy, the smaller the number of electrons that reaches the detector, but the more precisely is their energy determined. Behind the energy filter is the actual detector, which consists of an electron multiplier or a channeltron, which amplifies the incoming photoelectrons to measurable currents. Advanced hemispherical analyzers contain up to five multipliers. For further details of these instruments the interested reader should refer to other textbooks [20, 21]. 

The resolution of XPS is determined by the line width of the X-ray source, the broadening due to the analyzer, and the natural line width of the level under study. These three factors are related as follows  

The line width of the X-ray source is on the order of 1 eV for A1 or Mg Ka sources, but can be reduced to better than about 0.3 eV with the use of a monochromator. A monochromator contains a quartz crystal which is positioned at the correct Bragg angle for A1 Ka radiation. The monochromator not only narrows this line significantly and focuses it onto the sample, but also cuts out all unwanted X-ray satellites and background radiation. One important advantage of using a monochromator is that heat and secondary electrons generated by the X-ray source cannot reach the sample. 

The broadening due to the analyzer depends on the energy at which the electrons travel through the analyzer, and the width of the slits between the energy filter and the actual detector. The analyzer contribution to the line width becomes irrelevant at low pass energies, albeit at the cost of intensity. 

The natural line width is determined by Heisenberg s uncertainty relationship  

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