Page 428 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN012C-568 July 26, 2001 15:32

78 Photoelectron Spectroscopy

FIGURE 16 Sketch of the analyzer/detector system of the VG
® ®
ESCALAB 220iXL produced by VG Scientific .

Figure 17a shows the principle of a hemispherical ana-
lyzer. At a given voltage U A , electrons that enter through
the aperture A with a certain kinetic energy (the pass en-
ergy) can leave through the exit aperture B. The relation
between pass energy and U A is determined by the dis- FIGURE 17 Schematic cross section of (a) a hemispherical an-
tance D. If electrons that do not enter perpendicular to alyzer, (b) a cylindrical mirror analyzer with axis–axis-type focus-
the entrance aperture leave through the exit aperture, the ing, and (c) ring–axis-type focusing. 1 and 2—inner and outer
electrodes; A and B—aperture windows; R—receipt diaphragm.
analyzer is focusing. If focusing conditions are met only
within one plane the analyzer is called single focusing. If
the conditions are met in two dimensions the analyzer is
double focusing. The hemispherical analyzer is first-order fields become important. For this reason electrons are re-
double focusing, thus it accepts electrons from a solid an- tarded before they enter the deflection analyzer. This is
gle γ . The larger the γ , the higher the sensitivity at a given usually done in the last part of the lens system. However,
pass energy. decreasing the pass energy also decreases the sensitivity.
The resolution of the analyzer depends on its geometric Therefore modern instruments allow choosing of differ-
parameters, that is, the radius of the hemispheres, the area ent pass energies and thereby the selection of either the
of entrance and exit aperture, the angle γ , and the pass en- highest possible sensitivity or the highest possible resolu-
ergy. For a fixed γ , the resolution increases with decreas- tion or an appropriate compromise between the two. One
ing pass energy until a limit (usually a few eV) where stray consequence of Eq. (19) is that reducing the pass energy