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Monochromatic XPS
- some examples of the use of the new large backplane system
1. Performance at the Fermi edge of a metal Monochromatic X-ray sources provide a dual benefit of narrow excitation line widths for enhanced energy resolution, and the removal of the bremsstrahlung radiation for low background levels and minimum sample damage. They also provide the benefit of removing the satellites associated with non-monochromatic sources for improved peak identification and detection.
The monochromator used on the AXIS 165 and AXIS HSi instruments is described in brief detail below (Catalogue number WX-418).
Using the X-ray monochromator option on an AXIS instrument enables the highest possible energy resolution to be obtained. This applies both to conducting and insulating samples, where the integrated, patented, charge neutralization system provides a low energy uniform flux of electrons for efficient charge neutralization.
The figure below shows the performance obtained in the valence band region on a standard, sputter cleaned Ag sample. This performance is obtained using standard operating parameters (10eV pass energy, 450W X-ray power). A revealing test of the monochromator system performance is the resolution obtained on the Fermi edge. Using the standard measurement criteria of 16% to 84% rise, an edge resolution of <0.28eV is obtained. This measurement is made without any corrections for the Fermi-Dirac distribution, which would be appropriate since the measurement was made at room temperature.
2. Performance with insulating polymeric materials A particular application area where monochromatic sources offer a significant advantage is in the analysis of polymeric surfaces. These types of materials are frequently damaged by X-rays, or undergo other types of chemical reaction when exposed to the high X-ray energies generated by the non-monochromatic sources.
When using monochromatic X-rays however, the spectra can often be difficult to obtain due to surface charging effects. In order to observe high quality data under these circumstances, it is necessary to neutralize this charge carefully. On the AXIS 165, the integrated and patented charge neutralization system generates a flood of low energy electrons (typically 0.1eV). These ensure that surface charge is correctly compensated, facilitating acquisition of accurate high resolution spectra. This capability can be easily demonstrated using for example a PTFE (poly-tetra-fluoro ethylene) surface. Because of its highly insulating nature, this material will quickly accumulate a surface charge when exposed to monochromatic X-rays, making analysis difficult. The charge neutralizer on the AXIS instruments eliminates this surface charge build up in an efficient manner, enabling easy recording of high resolution information.
monochromatic X-rays. No special sample mounting techniques were used (such as a mask).
3. Some details of the monochromator
(Catalog Number WX-418)
The monochromator is based on the classical Johann-Johansson geometry, which has particular advantages for routine and research analysis.
Rowland Circle Geometry
The mechanical arrangement incorporates simplified "3-space" adjustment using independent orthogonal verniers, (should the device ever require resetting after installation) and cooling for the massively mounted torroidal backplane is available if required (although this is unnecessary in normal operation). The high power X-ray source also incorporates axial and "tilt" adjustment for complete optimisation of performance.
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 Copyright © Kratos 1998 Last Update: 98/4/14 Kratos Analytical is a wholly owned subsidiary of the Shimadzu Corporation. |
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