Damage photon beam

We shall concern ourselves here with the use of an X-ray probe as a surface analysis technique in X-ray photoelectron spectroscopy (XPS) also known as Electron Spectroscopy for Chemical Analysis (ESCA). High energy photons constitute the XPS probe, which are less damaging than an electron probe, therefore XPS is the favoured technique for the analysis of the surface chemistry of radiation sensitive materials. The X-ray probe has the disadvantage that, unlike an electron beam, it cannot be focussed to permit high spatial resolution imaging of the surface. 

Fig. 11.6. Diagram depicting desorption ionization (MALDI, FAB or SIMS). The operating principles of the three techniques are similar. The initiating event is exposure of the analyte to a beam of photons, atoms or ions. In order to prevent damage to the fragile analyte molecules and enhance the conversion of the involatile molecules into gas-phase ions, a matrix is employed. For MALDI, the matrix compounds are UV absorbing compounds such as hydroxycinnamic acid. The most commonly used FAB matrix was glycerol and ammonium chloride was employed by some investigators in SIMS experiments (although at low ion beam fluxes molecular species could be effectively ionized for many analytes with minimal evidence of damage by the primary ion beam).

Even more elegantly, the local resolution is improved by irradiation with very intense focused femtosecond laser pulses outside the absorption range of the fluoro-phore (e.g., in the near-infrared). The very intense focus of the laser beam—and only this—will excite the fluorophore by nonresonant two-photon absorption. Artifacts by scattered primary radiation are ruled out and the local resolution is comparable to a confocal microscope. In addition, the damage of the sample by laser light absorption is reduced to a minimum. 

They utilize X-ray diffraction. X-ray diffraction allows direct qualitative and quantitative phase characterization — even in multiphase regions — and no potentially perturbing additives or molecular labels are needed. Although the high photon flux of synchrotron radiation is potentially damaging to the sample [15], particular parts need only be exposed to the beam for a short period of time and as a result, radiation damage is not a problem with this method. 

Sample heating in SR beams as a consideration was introduced by Stuhrmann (1978) his example is cited in detail in section 6.5.4.1. Hel-liwell and Fourme (1983) considered radiation damage and sample heating in evaluating the usefulness of the prospective fluxes at the specimen that might be anticipated using the ESRF. Helliwell and Fourme (1983) and Helliwell (1984, pp. 1470-3) discuss the need to go to shorter X-ray wavelengths (e.g. 0.5 A), to reduce the fraction of absorbed photons, and to use cryotemperatures, with frozen crystals mounted on a copper fibre, to limit the temperature rise experienced by the sample. In this way, frozen microcrystals of protein of size lO/zm should be successfully studied on the ESRF. This application of the ESRF was further discussed in Helliwell (1989). 

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