XPS Techniques and Results

Presently, electrostatic energy analysers have fully replaced magnetic ones, since the former have many practical advantages. An electrostatic analyser consists essentially of two concentric cylindrical capacitor electrodes, that produce a well-defined radial electric field. 

In order to be able to measure many electron lines simultaneously a multichannel detector is frequently placed in the detector plane instead of using a detector behind an exit slit. The electrons then impinge on a micro-channel plate (see also Fig.6.38), in which electron multiplication occurs due to secondary emission by the inner wall material in the densely packed tubes in the plate. The original electron line image in the focal plane of the spectrometer is amplified and, by using two channel plates in series an electron multiplication of 10 can be obtained. The electron showers are converted into optical signals on a phosphor screen which is viewed by a diode array or vidicon (TV camera) (Fig.6.38). 

As an example of an atomic photo-electron spectrum a recording for mercury is shown in Fig.5.14, in which all populated levels in the N, O and P shells (n = 4, 5 and 6) can be observed. The common level designations from X-ray spectroscopy (Sect.5.1 Fig.4.8) have been used. Thus Nj corresponds to the 4s level, Njj and Njjj are the 4p 2 4p3 2 struc- 

A sulphur L-shell XPS spectrum showing valence orbitals of the SFg molecule is shown in Fig.5.15. An ultra-soft X-ray emission spectrum for the same molecule is also included. The figure illustrates how the different selection rules for X-ray emission and XPS lead to complementary information for the interpretation of the valence orbital structure of the molecule. 

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