Analyzers, electron energy retardation

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 electron energy selection is usually made through two different modes fixed analyzer transmission and fixed retardation ratio. The first one is the one used when a quantitative analysis is required. It consists of keeping the analyzer voltages constant, the kinetic energy being swept by a variable retardation voltage... 

In most electron spectroscopic analyses, the kinetic energies of the electrons entering the analyzer are retarded to either a constant energy or by a constant factor. These approaches lead to two modes of operation the constant analyzer energy (CAE) mode and the constant retard ratio (CRR) mode. 

The CRR mode involves retarding the electron kinetic energies to a constant ratio of H /H where H is the energy passed by the analyzer. Thus, the energies are retarded by a constant factor. Spectra acquired in this mode ate less easy to quantify, but small peaks at low kinetic energies ate readily detected. This mode of operation results in spectra of constant relative resolution throughout. The relative resolution is improved in this mode by a factor of E. ... 

The apparatuses used for the studies of both ammonia synthesis emd hydrodesulfurization were almost identical, consisting of a UHV chamber pumped by both ion and oil diffusion pumps to base pressures of 1 x10 " Torr. Each chamber was equipped with Low Energy Electron Diffraction optics used to determine the orientation of the surfaces and to ascertain that the surfaces were indeed well-ordered. The LEED optics doubled as retarding field analyzers used for Auger Electron Spectroscopy. In addition, each chamber was equipped with a UTI 100C quadrupole mass spectrometer used for analysis of background gases and for Thermal Desorption Spectroscopy studies. 

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