Retarding field analyzer

Fig. 3.3. Experimental arrangement used by Krauss and Gruen for SSIMS [3.8] a qua-drupole mass spectrometer was used for mass analysis and a retarding-field analyzer for prior energy selection (a) ion gun (b)-(d) lenses 1-3 (e) quadrupole mass spec-...

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. 

The potential energy diagram for an ion formed right at the critical distance of field ionization, xc (i.e. the most energetic ion), in a retarding potential analyzer is shown in Fig. 2.4. From this diagram, one obtains... 

The energies of the electrons being collected are filtered by applying different potentials to the inner and outer cylinders. The potentials of the cylinders focus the electrons to a plane where a slit aperture is located. The slit aperture controls the energy resolution of the analyzer and in the case where retardation fields are not employed, resolutions follow an E/AE relationship. Beyond the analyzer resolution adjustment aperture is the electron detector. Modem Auger systems make use of multiple detectors in order to increase the electron counting efficiency of the instmment in a similar fashion to those found in XPS systems. 

The combination of retardation and deflection allows two different modes of operation. In the constant analyzer energy (CAE) mode the pass energy is kept constant and the spectrum is scanned by a variation of the retarding field. In this mode the resolution AEa is kept constant over the whole spectrum. In the constant retard ratio mode (CRR) the voltage difference between the hemispheres is proportional to the retarding field. In this mode the relative resolution AEjE is kept constant. The CAE mode is normally used for PES, the CRR mode for Auger spectroscopy. 

---