Electron emission photon beams

Electron Emission from Surfaces by Incident Electron or Photon Beams... 

ELECTRON EMISSION FROM SURFACES BY INCIDENT ELECTRON OR PHOTON BEAMS... 

Nearly all these techniques involve interrogation of the surface with a particle probe. The function of the probe is to excite surface atoms into states giving rise to emission of one or more of a variety of secondary particles such as electrons, photons, positive and secondary ions, and neutrals. Because the primary particles used in the probing beam can also be electrons or photons, or ions or neutrals, many separate techniques are possible, each based on a different primary-secondary particle combination. Most of these possibilities have now been established, but in fact not all the resulting techniques are of general application, some because of the restricted or specialized nature of the information obtained and others because of difficult experimental requirements. In this publication, therefore, most space is devoted to those surface analytical techniques that are widely applied and readily available commercially, whereas much briefer descriptions are given of the many others the use of which is less common but which - in appropriate circumstances, particularly in basic research - can provide vital information. 

When the accelerating voltage reaches a specific value (dependent on the nature of the target material), the electrons from the beam are capable of knocking out core-level electrons from the target material, thus giving rise to core vacancies. These are quickly filled by electrons in upper levels and this results in the emission of X-ray photons of characteristic energies which depend on the... 

Figure 4.43 Energy- and angle-resolved triple-differential cross section for direct double photoionization in helium at 99 eV photon energy. The diagram shows the polar plot of relative intensity values for one electron (ea) kept at a fixed position while the angle of the coincident electron (eb) is varied. The data refer to electron emission in a plane perpendicular to the photon beam direction for partially linearly polarized light (Stokes parameter = 0.554) and for equal energy sharing of the excess energy, i.e., a = b = 10 eV. Experimental data are given by points with error bars, theoretical data by the solid curve.

Figure 4.45 Illustration of the content of equ. (4.90) which describes the angular distribution of Auger electrons (eb) in coincidence with the preceding photoelectron (ea). The data refer to 2p3/2 ionization of magnesium by linearly polarized photons of 80 eV and subsequent L3-M1M1 Auger decay, with emission of both electrons in a plane perpendicular to the photon beam direction. The alignment tensor a,

Figure 4.46 Energy- and angle-resolved patterns for two-electron emission in the two-step process of 2p3/2 photoionization of magnesium with subsequent L3-M, M, Auger decay induced by 80 eV photons with linear polarization (electric field vector along the x-axis). Both electrons are detected in a plane perpendicular to the photon beam direction the direction of the photoelectron (ea) is fixed at ( ) a = 180° and (b) = 150°, while the...

The most essential plasma device characteristics that are needed in order to obtain impurity release rates are the fluxes of photons and of charged and neutral particles to the wall. It will be necessary to have detailed information on the energy spectra and fluxes to walls, limiters and beam dumps of thermal electrons and ions, photons, a-particles, runaway electrons, charge exchange neutrals, neutral beam and impurity neutrals and ions. The effects of sheath potentials, secondary electron emission and unipolar arcing need to be included in these calculations. 

Fine structure experiments are often carried out with synchrotron sources, since the initial electron state is better defined for photoemission than for electron excitation. When core-hole decay is detected by Auger or secondary electron emission, the technique is surface sensitive. Core-hole decay can also be detected by fluorescence, or by adsorption of the incident photon beam. These methods are not intrinsically surface sensitive, but they are useful when the source atoms are exclusively located at the surface. 

Beam fill errors can create stray electrons that are temporally separated from the main bunch, leading to detection of electronically scattered photons within the time interval where delayed emission from the nuclear excitation is expected. This signal will vary weakly across the narrow energy window of a NRVS scan, and thus contribute to a uniform background. In many cases, adjustments to the width and delay time of the gate signal can eliminate this contribution to background. 

Let us look briefly at the principle of the technique, illustrated in Figure 5.1. The sample to be studied is bombarded by an X-ray photon beam. Usually, the Ka emission of A1 (energy Av = 1486,6 eV) or Mg (/rv = 1253.6 cV) are used. Under the effect of this impact, electrons from atomic energy levels of the different elements are emitted and analysed in terms of number and energy by an appropriate detector. The measured kinetic energy is directly related to the electron binding energy, various orbitals involved ... 

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